Proving a result by making discriminant zero$y =f(x) =(ax^2 + bx +c)/(dx^2+ex+f)$ We have to find the conditions for this it takes all real values.Equation with the discriminantWhat is $textitthe$ discriminant of a degree $n$ polynomial?Question on integral values of a quadraticDifference between “two equal roots” and “one root”?Theory of Equations and co primeWhere am I making the mistake?Intuitive meaning behind the DiscriminantWhat causes the extraneous root when intersecting parabola $y^2=4ax$ with circle $x^2+y^2=9a^2/4$?

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Proving a result by making discriminant zero


$y =f(x) =(ax^2 + bx +c)/(dx^2+ex+f)$ We have to find the conditions for this it takes all real values.Equation with the discriminantWhat is $textitthe$ discriminant of a degree $n$ polynomial?Question on integral values of a quadraticDifference between “two equal roots” and “one root”?Theory of Equations and co primeWhere am I making the mistake?Intuitive meaning behind the DiscriminantWhat causes the extraneous root when intersecting parabola $y^2=4ax$ with circle $x^2+y^2=9a^2/4$?






.everyoneloves__top-leaderboard:empty,.everyoneloves__mid-leaderboard:empty,.everyoneloves__bot-mid-leaderboard:empty margin-bottom:0;








6












$begingroup$



If the roots of given Quadratic equation
$$a(b-c)x^2 +b(c-a)x + c(a-b)=0$$
are equal, prove the following:
$$frac2b=frac1a+frac1c$$.




MY approach:



Method 1: put Discriminant=0 and get stuck.



Method2:add $acx$ on both sides and get $(x-1)$ as factor ,so other root is also 1 and hence the result.

But if someone can prove the result by making discriminant zero (method 1), that would be more rigorous.
Thank you.










share|cite|improve this question











$endgroup$









  • 1




    $begingroup$
    I appreciate you trying to work it directly using discriminant, but your method2 is rly clever!
    $endgroup$
    – ganeshie8
    8 hours ago











  • $begingroup$
    @ganeshie8 grace of ganesh hehehe
    $endgroup$
    – George Carlin
    8 hours ago

















6












$begingroup$



If the roots of given Quadratic equation
$$a(b-c)x^2 +b(c-a)x + c(a-b)=0$$
are equal, prove the following:
$$frac2b=frac1a+frac1c$$.




MY approach:



Method 1: put Discriminant=0 and get stuck.



Method2:add $acx$ on both sides and get $(x-1)$ as factor ,so other root is also 1 and hence the result.

But if someone can prove the result by making discriminant zero (method 1), that would be more rigorous.
Thank you.










share|cite|improve this question











$endgroup$









  • 1




    $begingroup$
    I appreciate you trying to work it directly using discriminant, but your method2 is rly clever!
    $endgroup$
    – ganeshie8
    8 hours ago











  • $begingroup$
    @ganeshie8 grace of ganesh hehehe
    $endgroup$
    – George Carlin
    8 hours ago













6












6








6


3



$begingroup$



If the roots of given Quadratic equation
$$a(b-c)x^2 +b(c-a)x + c(a-b)=0$$
are equal, prove the following:
$$frac2b=frac1a+frac1c$$.




MY approach:



Method 1: put Discriminant=0 and get stuck.



Method2:add $acx$ on both sides and get $(x-1)$ as factor ,so other root is also 1 and hence the result.

But if someone can prove the result by making discriminant zero (method 1), that would be more rigorous.
Thank you.










share|cite|improve this question











$endgroup$





If the roots of given Quadratic equation
$$a(b-c)x^2 +b(c-a)x + c(a-b)=0$$
are equal, prove the following:
$$frac2b=frac1a+frac1c$$.




MY approach:



Method 1: put Discriminant=0 and get stuck.



Method2:add $acx$ on both sides and get $(x-1)$ as factor ,so other root is also 1 and hence the result.

But if someone can prove the result by making discriminant zero (method 1), that would be more rigorous.
Thank you.







algebra-precalculus quadratics






share|cite|improve this question















share|cite|improve this question













share|cite|improve this question




share|cite|improve this question








edited 8 hours ago









Robert Z

112k10 gold badges79 silver badges153 bronze badges




112k10 gold badges79 silver badges153 bronze badges










asked 8 hours ago









George CarlinGeorge Carlin

1445 bronze badges




1445 bronze badges










  • 1




    $begingroup$
    I appreciate you trying to work it directly using discriminant, but your method2 is rly clever!
    $endgroup$
    – ganeshie8
    8 hours ago











  • $begingroup$
    @ganeshie8 grace of ganesh hehehe
    $endgroup$
    – George Carlin
    8 hours ago












  • 1




    $begingroup$
    I appreciate you trying to work it directly using discriminant, but your method2 is rly clever!
    $endgroup$
    – ganeshie8
    8 hours ago











  • $begingroup$
    @ganeshie8 grace of ganesh hehehe
    $endgroup$
    – George Carlin
    8 hours ago







1




1




$begingroup$
I appreciate you trying to work it directly using discriminant, but your method2 is rly clever!
$endgroup$
– ganeshie8
8 hours ago





$begingroup$
I appreciate you trying to work it directly using discriminant, but your method2 is rly clever!
$endgroup$
– ganeshie8
8 hours ago













$begingroup$
@ganeshie8 grace of ganesh hehehe
$endgroup$
– George Carlin
8 hours ago




$begingroup$
@ganeshie8 grace of ganesh hehehe
$endgroup$
– George Carlin
8 hours ago










3 Answers
3






active

oldest

votes


















9














$begingroup$

Following your first approach, we have that the discriminant is
$$beginalign
Delta&=b^2(c−a)^2-4a(b−c)c(a−b)\
&=b^2(c−a)^2+4ac(b^2-b(a+c)+ac)\
&=b^2(c+a)^2+(2ac)^2-2b(a+c)(2ac)\
&=(b(a+c)-2ac)^2
endalign.$$

and by letting $Delta=0$, it is easy to show that $frac2b=frac1a+frac1c$.



As regards your second approach, since you already noted that $x=1$ is a root, we have a double root if and only if the following polynomial identity holds (polynomial factorization is unique):
$$a(b-c)x^2 +b(c-a)x + c(a-b)=0=a(b-c)(x-1)^2.$$
Now by letting $x=0$ we find $$c(a-b)=a(b-c)$$ which implies $frac2b=frac1a+frac1c$.






share|cite|improve this answer











$endgroup$














  • $begingroup$
    How did you get that in just one step? I mean... is it an obvious identity or result?
    $endgroup$
    – ab123
    8 hours ago










  • $begingroup$
    @ab123 I was actually writing up something more or less along these lines. I didn't bother doing the tedious stuff, I just dumped the quadratic into Wolfram, told it to find the discriminant, and lo and behold one of the alternative forms worked: link
    $endgroup$
    – Eevee Trainer
    8 hours ago











  • $begingroup$
    @EeveeTrainer wow! Wolfram is quite intelligent
    $endgroup$
    – ab123
    8 hours ago






  • 1




    $begingroup$
    @GeorgeCarlin Because $a(b-c)x^2 +b(c-a)x + c(a-b)=0=a(b-c)(x-1)^2$ is a polynomial identity which holds for any $x$. I edited my answer.
    $endgroup$
    – Robert Z
    7 hours ago







  • 1




    $begingroup$
    What you wrote is not an identity. The identity is $$ a(b-c)x^2+b(c-a)x+c(a-b)-a(b-c)(x-1)^2=(x-1)(ab+bc-2ac)$$ for all $,a,b,c,x.,$ Now if the left side is $0$ and $,xne 1,$ then we get $,ab+bc-2ac=0.$
    $endgroup$
    – Somos
    3 hours ago



















2














$begingroup$

If you label:
$$begincasesa(b-c)=p\
b(c-a)=q\
c(a-b)=rendcases Rightarrow p+q+r=0$$

then the equation becomes:
$$px^2+qx+r=0,\
D=q^2-4pr=(-p-r)^2-4pr=(p-r)^2=0 Rightarrow p=rRightarrow \
a(b-c)=c(a-b) Rightarrow 2ac=ab+bc stackrelcdot frac1abcRightarrow frac2b=frac1a+frac1c.$$






share|cite|improve this answer









$endgroup$






















    1














    $begingroup$

    Since $a(b-c)+b(c-a)+c(a-b)=0$, you know that $x=1$ is a root, as you observed.



    With synthetic division by $x-1$, we get, assuming $ane0$ and $bne c$,
    $$
    beginarraycc
    & ab-ac & bc-ab & ac-bc \
    1 & downarrow & ab-ac & bc-ac \
    hline
    & ab-ac & bc-ac & 0
    endarray
    $$

    The other root is
    $$
    -fracbc-acab-ac
    $$

    Therefore $bc-ac=-ab+ac$ is the condition that the equation has two equal roots. This becomes
    $$
    2ac=bc+ab
    $$

    and, dividing by $abc$, we get
    $$
    frac2b=frac1a+frac1c
    $$



    If $a=0$ or $b=c$ the equation is not a quadratic. Can it be $b=0$? In this case we get $-acx^2+ac=0$ and we need $cne0$; the roots are $1$ and $-1$. With $c=0$ it would be the same.






    share|cite|improve this answer









    $endgroup$

















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      3 Answers
      3






      active

      oldest

      votes








      3 Answers
      3






      active

      oldest

      votes









      active

      oldest

      votes






      active

      oldest

      votes









      9














      $begingroup$

      Following your first approach, we have that the discriminant is
      $$beginalign
      Delta&=b^2(c−a)^2-4a(b−c)c(a−b)\
      &=b^2(c−a)^2+4ac(b^2-b(a+c)+ac)\
      &=b^2(c+a)^2+(2ac)^2-2b(a+c)(2ac)\
      &=(b(a+c)-2ac)^2
      endalign.$$

      and by letting $Delta=0$, it is easy to show that $frac2b=frac1a+frac1c$.



      As regards your second approach, since you already noted that $x=1$ is a root, we have a double root if and only if the following polynomial identity holds (polynomial factorization is unique):
      $$a(b-c)x^2 +b(c-a)x + c(a-b)=0=a(b-c)(x-1)^2.$$
      Now by letting $x=0$ we find $$c(a-b)=a(b-c)$$ which implies $frac2b=frac1a+frac1c$.






      share|cite|improve this answer











      $endgroup$














      • $begingroup$
        How did you get that in just one step? I mean... is it an obvious identity or result?
        $endgroup$
        – ab123
        8 hours ago










      • $begingroup$
        @ab123 I was actually writing up something more or less along these lines. I didn't bother doing the tedious stuff, I just dumped the quadratic into Wolfram, told it to find the discriminant, and lo and behold one of the alternative forms worked: link
        $endgroup$
        – Eevee Trainer
        8 hours ago











      • $begingroup$
        @EeveeTrainer wow! Wolfram is quite intelligent
        $endgroup$
        – ab123
        8 hours ago






      • 1




        $begingroup$
        @GeorgeCarlin Because $a(b-c)x^2 +b(c-a)x + c(a-b)=0=a(b-c)(x-1)^2$ is a polynomial identity which holds for any $x$. I edited my answer.
        $endgroup$
        – Robert Z
        7 hours ago







      • 1




        $begingroup$
        What you wrote is not an identity. The identity is $$ a(b-c)x^2+b(c-a)x+c(a-b)-a(b-c)(x-1)^2=(x-1)(ab+bc-2ac)$$ for all $,a,b,c,x.,$ Now if the left side is $0$ and $,xne 1,$ then we get $,ab+bc-2ac=0.$
        $endgroup$
        – Somos
        3 hours ago
















      9














      $begingroup$

      Following your first approach, we have that the discriminant is
      $$beginalign
      Delta&=b^2(c−a)^2-4a(b−c)c(a−b)\
      &=b^2(c−a)^2+4ac(b^2-b(a+c)+ac)\
      &=b^2(c+a)^2+(2ac)^2-2b(a+c)(2ac)\
      &=(b(a+c)-2ac)^2
      endalign.$$

      and by letting $Delta=0$, it is easy to show that $frac2b=frac1a+frac1c$.



      As regards your second approach, since you already noted that $x=1$ is a root, we have a double root if and only if the following polynomial identity holds (polynomial factorization is unique):
      $$a(b-c)x^2 +b(c-a)x + c(a-b)=0=a(b-c)(x-1)^2.$$
      Now by letting $x=0$ we find $$c(a-b)=a(b-c)$$ which implies $frac2b=frac1a+frac1c$.






      share|cite|improve this answer











      $endgroup$














      • $begingroup$
        How did you get that in just one step? I mean... is it an obvious identity or result?
        $endgroup$
        – ab123
        8 hours ago










      • $begingroup$
        @ab123 I was actually writing up something more or less along these lines. I didn't bother doing the tedious stuff, I just dumped the quadratic into Wolfram, told it to find the discriminant, and lo and behold one of the alternative forms worked: link
        $endgroup$
        – Eevee Trainer
        8 hours ago











      • $begingroup$
        @EeveeTrainer wow! Wolfram is quite intelligent
        $endgroup$
        – ab123
        8 hours ago






      • 1




        $begingroup$
        @GeorgeCarlin Because $a(b-c)x^2 +b(c-a)x + c(a-b)=0=a(b-c)(x-1)^2$ is a polynomial identity which holds for any $x$. I edited my answer.
        $endgroup$
        – Robert Z
        7 hours ago







      • 1




        $begingroup$
        What you wrote is not an identity. The identity is $$ a(b-c)x^2+b(c-a)x+c(a-b)-a(b-c)(x-1)^2=(x-1)(ab+bc-2ac)$$ for all $,a,b,c,x.,$ Now if the left side is $0$ and $,xne 1,$ then we get $,ab+bc-2ac=0.$
        $endgroup$
        – Somos
        3 hours ago














      9














      9










      9







      $begingroup$

      Following your first approach, we have that the discriminant is
      $$beginalign
      Delta&=b^2(c−a)^2-4a(b−c)c(a−b)\
      &=b^2(c−a)^2+4ac(b^2-b(a+c)+ac)\
      &=b^2(c+a)^2+(2ac)^2-2b(a+c)(2ac)\
      &=(b(a+c)-2ac)^2
      endalign.$$

      and by letting $Delta=0$, it is easy to show that $frac2b=frac1a+frac1c$.



      As regards your second approach, since you already noted that $x=1$ is a root, we have a double root if and only if the following polynomial identity holds (polynomial factorization is unique):
      $$a(b-c)x^2 +b(c-a)x + c(a-b)=0=a(b-c)(x-1)^2.$$
      Now by letting $x=0$ we find $$c(a-b)=a(b-c)$$ which implies $frac2b=frac1a+frac1c$.






      share|cite|improve this answer











      $endgroup$



      Following your first approach, we have that the discriminant is
      $$beginalign
      Delta&=b^2(c−a)^2-4a(b−c)c(a−b)\
      &=b^2(c−a)^2+4ac(b^2-b(a+c)+ac)\
      &=b^2(c+a)^2+(2ac)^2-2b(a+c)(2ac)\
      &=(b(a+c)-2ac)^2
      endalign.$$

      and by letting $Delta=0$, it is easy to show that $frac2b=frac1a+frac1c$.



      As regards your second approach, since you already noted that $x=1$ is a root, we have a double root if and only if the following polynomial identity holds (polynomial factorization is unique):
      $$a(b-c)x^2 +b(c-a)x + c(a-b)=0=a(b-c)(x-1)^2.$$
      Now by letting $x=0$ we find $$c(a-b)=a(b-c)$$ which implies $frac2b=frac1a+frac1c$.







      share|cite|improve this answer














      share|cite|improve this answer



      share|cite|improve this answer








      edited 7 hours ago

























      answered 8 hours ago









      Robert ZRobert Z

      112k10 gold badges79 silver badges153 bronze badges




      112k10 gold badges79 silver badges153 bronze badges














      • $begingroup$
        How did you get that in just one step? I mean... is it an obvious identity or result?
        $endgroup$
        – ab123
        8 hours ago










      • $begingroup$
        @ab123 I was actually writing up something more or less along these lines. I didn't bother doing the tedious stuff, I just dumped the quadratic into Wolfram, told it to find the discriminant, and lo and behold one of the alternative forms worked: link
        $endgroup$
        – Eevee Trainer
        8 hours ago











      • $begingroup$
        @EeveeTrainer wow! Wolfram is quite intelligent
        $endgroup$
        – ab123
        8 hours ago






      • 1




        $begingroup$
        @GeorgeCarlin Because $a(b-c)x^2 +b(c-a)x + c(a-b)=0=a(b-c)(x-1)^2$ is a polynomial identity which holds for any $x$. I edited my answer.
        $endgroup$
        – Robert Z
        7 hours ago







      • 1




        $begingroup$
        What you wrote is not an identity. The identity is $$ a(b-c)x^2+b(c-a)x+c(a-b)-a(b-c)(x-1)^2=(x-1)(ab+bc-2ac)$$ for all $,a,b,c,x.,$ Now if the left side is $0$ and $,xne 1,$ then we get $,ab+bc-2ac=0.$
        $endgroup$
        – Somos
        3 hours ago

















      • $begingroup$
        How did you get that in just one step? I mean... is it an obvious identity or result?
        $endgroup$
        – ab123
        8 hours ago










      • $begingroup$
        @ab123 I was actually writing up something more or less along these lines. I didn't bother doing the tedious stuff, I just dumped the quadratic into Wolfram, told it to find the discriminant, and lo and behold one of the alternative forms worked: link
        $endgroup$
        – Eevee Trainer
        8 hours ago











      • $begingroup$
        @EeveeTrainer wow! Wolfram is quite intelligent
        $endgroup$
        – ab123
        8 hours ago






      • 1




        $begingroup$
        @GeorgeCarlin Because $a(b-c)x^2 +b(c-a)x + c(a-b)=0=a(b-c)(x-1)^2$ is a polynomial identity which holds for any $x$. I edited my answer.
        $endgroup$
        – Robert Z
        7 hours ago







      • 1




        $begingroup$
        What you wrote is not an identity. The identity is $$ a(b-c)x^2+b(c-a)x+c(a-b)-a(b-c)(x-1)^2=(x-1)(ab+bc-2ac)$$ for all $,a,b,c,x.,$ Now if the left side is $0$ and $,xne 1,$ then we get $,ab+bc-2ac=0.$
        $endgroup$
        – Somos
        3 hours ago
















      $begingroup$
      How did you get that in just one step? I mean... is it an obvious identity or result?
      $endgroup$
      – ab123
      8 hours ago




      $begingroup$
      How did you get that in just one step? I mean... is it an obvious identity or result?
      $endgroup$
      – ab123
      8 hours ago












      $begingroup$
      @ab123 I was actually writing up something more or less along these lines. I didn't bother doing the tedious stuff, I just dumped the quadratic into Wolfram, told it to find the discriminant, and lo and behold one of the alternative forms worked: link
      $endgroup$
      – Eevee Trainer
      8 hours ago





      $begingroup$
      @ab123 I was actually writing up something more or less along these lines. I didn't bother doing the tedious stuff, I just dumped the quadratic into Wolfram, told it to find the discriminant, and lo and behold one of the alternative forms worked: link
      $endgroup$
      – Eevee Trainer
      8 hours ago













      $begingroup$
      @EeveeTrainer wow! Wolfram is quite intelligent
      $endgroup$
      – ab123
      8 hours ago




      $begingroup$
      @EeveeTrainer wow! Wolfram is quite intelligent
      $endgroup$
      – ab123
      8 hours ago




      1




      1




      $begingroup$
      @GeorgeCarlin Because $a(b-c)x^2 +b(c-a)x + c(a-b)=0=a(b-c)(x-1)^2$ is a polynomial identity which holds for any $x$. I edited my answer.
      $endgroup$
      – Robert Z
      7 hours ago





      $begingroup$
      @GeorgeCarlin Because $a(b-c)x^2 +b(c-a)x + c(a-b)=0=a(b-c)(x-1)^2$ is a polynomial identity which holds for any $x$. I edited my answer.
      $endgroup$
      – Robert Z
      7 hours ago





      1




      1




      $begingroup$
      What you wrote is not an identity. The identity is $$ a(b-c)x^2+b(c-a)x+c(a-b)-a(b-c)(x-1)^2=(x-1)(ab+bc-2ac)$$ for all $,a,b,c,x.,$ Now if the left side is $0$ and $,xne 1,$ then we get $,ab+bc-2ac=0.$
      $endgroup$
      – Somos
      3 hours ago





      $begingroup$
      What you wrote is not an identity. The identity is $$ a(b-c)x^2+b(c-a)x+c(a-b)-a(b-c)(x-1)^2=(x-1)(ab+bc-2ac)$$ for all $,a,b,c,x.,$ Now if the left side is $0$ and $,xne 1,$ then we get $,ab+bc-2ac=0.$
      $endgroup$
      – Somos
      3 hours ago














      2














      $begingroup$

      If you label:
      $$begincasesa(b-c)=p\
      b(c-a)=q\
      c(a-b)=rendcases Rightarrow p+q+r=0$$

      then the equation becomes:
      $$px^2+qx+r=0,\
      D=q^2-4pr=(-p-r)^2-4pr=(p-r)^2=0 Rightarrow p=rRightarrow \
      a(b-c)=c(a-b) Rightarrow 2ac=ab+bc stackrelcdot frac1abcRightarrow frac2b=frac1a+frac1c.$$






      share|cite|improve this answer









      $endgroup$



















        2














        $begingroup$

        If you label:
        $$begincasesa(b-c)=p\
        b(c-a)=q\
        c(a-b)=rendcases Rightarrow p+q+r=0$$

        then the equation becomes:
        $$px^2+qx+r=0,\
        D=q^2-4pr=(-p-r)^2-4pr=(p-r)^2=0 Rightarrow p=rRightarrow \
        a(b-c)=c(a-b) Rightarrow 2ac=ab+bc stackrelcdot frac1abcRightarrow frac2b=frac1a+frac1c.$$






        share|cite|improve this answer









        $endgroup$

















          2














          2










          2







          $begingroup$

          If you label:
          $$begincasesa(b-c)=p\
          b(c-a)=q\
          c(a-b)=rendcases Rightarrow p+q+r=0$$

          then the equation becomes:
          $$px^2+qx+r=0,\
          D=q^2-4pr=(-p-r)^2-4pr=(p-r)^2=0 Rightarrow p=rRightarrow \
          a(b-c)=c(a-b) Rightarrow 2ac=ab+bc stackrelcdot frac1abcRightarrow frac2b=frac1a+frac1c.$$






          share|cite|improve this answer









          $endgroup$



          If you label:
          $$begincasesa(b-c)=p\
          b(c-a)=q\
          c(a-b)=rendcases Rightarrow p+q+r=0$$

          then the equation becomes:
          $$px^2+qx+r=0,\
          D=q^2-4pr=(-p-r)^2-4pr=(p-r)^2=0 Rightarrow p=rRightarrow \
          a(b-c)=c(a-b) Rightarrow 2ac=ab+bc stackrelcdot frac1abcRightarrow frac2b=frac1a+frac1c.$$







          share|cite|improve this answer












          share|cite|improve this answer



          share|cite|improve this answer










          answered 6 hours ago









          farruhotafarruhota

          25.4k2 gold badges10 silver badges46 bronze badges




          25.4k2 gold badges10 silver badges46 bronze badges
























              1














              $begingroup$

              Since $a(b-c)+b(c-a)+c(a-b)=0$, you know that $x=1$ is a root, as you observed.



              With synthetic division by $x-1$, we get, assuming $ane0$ and $bne c$,
              $$
              beginarraycc
              & ab-ac & bc-ab & ac-bc \
              1 & downarrow & ab-ac & bc-ac \
              hline
              & ab-ac & bc-ac & 0
              endarray
              $$

              The other root is
              $$
              -fracbc-acab-ac
              $$

              Therefore $bc-ac=-ab+ac$ is the condition that the equation has two equal roots. This becomes
              $$
              2ac=bc+ab
              $$

              and, dividing by $abc$, we get
              $$
              frac2b=frac1a+frac1c
              $$



              If $a=0$ or $b=c$ the equation is not a quadratic. Can it be $b=0$? In this case we get $-acx^2+ac=0$ and we need $cne0$; the roots are $1$ and $-1$. With $c=0$ it would be the same.






              share|cite|improve this answer









              $endgroup$



















                1














                $begingroup$

                Since $a(b-c)+b(c-a)+c(a-b)=0$, you know that $x=1$ is a root, as you observed.



                With synthetic division by $x-1$, we get, assuming $ane0$ and $bne c$,
                $$
                beginarraycc
                & ab-ac & bc-ab & ac-bc \
                1 & downarrow & ab-ac & bc-ac \
                hline
                & ab-ac & bc-ac & 0
                endarray
                $$

                The other root is
                $$
                -fracbc-acab-ac
                $$

                Therefore $bc-ac=-ab+ac$ is the condition that the equation has two equal roots. This becomes
                $$
                2ac=bc+ab
                $$

                and, dividing by $abc$, we get
                $$
                frac2b=frac1a+frac1c
                $$



                If $a=0$ or $b=c$ the equation is not a quadratic. Can it be $b=0$? In this case we get $-acx^2+ac=0$ and we need $cne0$; the roots are $1$ and $-1$. With $c=0$ it would be the same.






                share|cite|improve this answer









                $endgroup$

















                  1














                  1










                  1







                  $begingroup$

                  Since $a(b-c)+b(c-a)+c(a-b)=0$, you know that $x=1$ is a root, as you observed.



                  With synthetic division by $x-1$, we get, assuming $ane0$ and $bne c$,
                  $$
                  beginarraycc
                  & ab-ac & bc-ab & ac-bc \
                  1 & downarrow & ab-ac & bc-ac \
                  hline
                  & ab-ac & bc-ac & 0
                  endarray
                  $$

                  The other root is
                  $$
                  -fracbc-acab-ac
                  $$

                  Therefore $bc-ac=-ab+ac$ is the condition that the equation has two equal roots. This becomes
                  $$
                  2ac=bc+ab
                  $$

                  and, dividing by $abc$, we get
                  $$
                  frac2b=frac1a+frac1c
                  $$



                  If $a=0$ or $b=c$ the equation is not a quadratic. Can it be $b=0$? In this case we get $-acx^2+ac=0$ and we need $cne0$; the roots are $1$ and $-1$. With $c=0$ it would be the same.






                  share|cite|improve this answer









                  $endgroup$



                  Since $a(b-c)+b(c-a)+c(a-b)=0$, you know that $x=1$ is a root, as you observed.



                  With synthetic division by $x-1$, we get, assuming $ane0$ and $bne c$,
                  $$
                  beginarraycc
                  & ab-ac & bc-ab & ac-bc \
                  1 & downarrow & ab-ac & bc-ac \
                  hline
                  & ab-ac & bc-ac & 0
                  endarray
                  $$

                  The other root is
                  $$
                  -fracbc-acab-ac
                  $$

                  Therefore $bc-ac=-ab+ac$ is the condition that the equation has two equal roots. This becomes
                  $$
                  2ac=bc+ab
                  $$

                  and, dividing by $abc$, we get
                  $$
                  frac2b=frac1a+frac1c
                  $$



                  If $a=0$ or $b=c$ the equation is not a quadratic. Can it be $b=0$? In this case we get $-acx^2+ac=0$ and we need $cne0$; the roots are $1$ and $-1$. With $c=0$ it would be the same.







                  share|cite|improve this answer












                  share|cite|improve this answer



                  share|cite|improve this answer










                  answered 7 hours ago









                  egregegreg

                  192k14 gold badges91 silver badges217 bronze badges




                  192k14 gold badges91 silver badges217 bronze badges































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