Difference of cubic vieta's

Algebra Level 3

Let a , b , a,b, and c c be the roots of the equation 2 x 3 x 2 + x + 3 = 0 2x^{3}-x^{2}+x+3=0 . Find the value of the expression below.

a 3 b 3 a b + b 3 c 3 b c + c 3 a 3 c a \large \frac{a^{3}-b^{3}}{a-b}+\frac{b^{3}-c^{3}}{b-c}+\frac{c^{3}-a^{3}}{c-a}


The answer is -1.

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3 solutions

Nihar Mahajan
May 3, 2015

Using Vieta's formula ,

a + b + c = 1 2 a b + b c + a c = 1 2 a+b+c=\dfrac{1}{2} \\ ab+bc+ac = \dfrac{1}{2}

Using Newton's identity ,

a 2 + b 2 + c 2 = ( a + b + c ) 2 2 ( a b + b c + a c ) = ( 1 2 ) 2 2 ( 1 2 ) = 3 4 a^2+b^2+c^2 = (a+b+c)^2 - 2(ab+bc+ac) = \left(\dfrac{1}{2}\right)^2 - 2\left(\dfrac{1}{2}\right) = \dfrac{-3}{4}

By using identity x 3 y 3 = ( x y ) ( x 2 + x y + y 2 ) x^3-y^3=(x-y)(x^2+xy+y^2) ,the terms like ( a b ) , ( b c ) , ( c a ) (a-b) , (b-c),(c-a) get cancelled ( a b c ) (a \neq b \neq c) and thus we can write the given expression as :

a 2 + a b + b 2 + b 2 + b c + c 2 + c 2 + a c + a 2 = 2 ( a 2 + b 2 + c 2 ) + a b + b c + a c = 2 ( 3 4 ) + 1 2 = 1 a^2+ab+b^2+b^2+bc+c^2+c^2+ac+a^2 \\ = 2(a^2+b^2+c^2)+ab+bc+ac \\ = 2\left(\dfrac{-3}{4}\right) + \dfrac{1}{2} \\ =\Large\boxed{-1}

The answer is -1, not 1. It's 2 ( a 2 + b 2 + c 2 ) + a b + a c + b c 2(a^2+b^2+c^2)+ab+ac+bc .

Pi Han Goh - 6 years, 1 month ago

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Sorry I had typo mistake

Nihar Mahajan - 6 years, 1 month ago

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@Nihar Mahajan , A typo is a mistake. There is nothing Known as Typo Mistake :3

Mehul Arora - 6 years, 1 month ago

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@Mehul Arora Well , if I had written this solution on paper with my "hand" such "typo" mistake would never happen.

Nihar Mahajan - 6 years, 1 month ago

BTW @Pi Han Goh Can you please help in this note . Thanks,

Nihar Mahajan - 6 years, 1 month ago

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I'm not quite good at writing proofs in geometry. Lemme reshare that!

Pi Han Goh - 6 years, 1 month ago
Noel Lo
May 4, 2015

Simplify the given expression to get 2 ( a 2 + b 2 + c 2 ) + a b + b c + a c 2(a^2 + b^2 + c^2) + ab +bc+ac . Since a 2 + b 2 + c 2 = ( a + b + c ) 2 2 ( a b + b c + a c ) a^2+b^2+c^2 = (a+b+c)^2 - 2(ab+bc+ac) , we have

2 ( a 2 + b 2 + c 2 ) + a b + b c + a c 2(a^2 + b^2 + c^2) + ab +bc+ac

= 2 [ ( a + b + c ) 2 2 ( a b + b c + a c ) ] + a b + b c + a c = 2[(a+b+c)^2 - 2(ab+bc+ac)] + ab+bc+ac

= 2 ( a + b + c ) 2 4 ( a b + b c + a c ) + a b + b c + a c =2(a+b+c)^2 - 4(ab+bc+ac)+ab+bc+ac

= 2 ( a + b + c ) 2 3 ( a b + b c + a c ) =2(a+b+c)^2 - 3(ab+bc+ac) . Substitute in the values and we are done.

What values? I think it is 2 ( a + b + c ) 2 2 (a+b+c)^{2}

Joel Tan - 6 years, 1 month ago

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Sorry. Thanks for pointing it out! I have updated it! Values as in a+b+c and ab+bc+ac which can be deduced easily from the cubic equation.

Noel Lo - 6 years, 1 month ago
Rajen Kapur
May 3, 2015

Cancelling out from numerator and denominator common factors, the expression reduces to 2 a 2 + a b 2\sum {a}^2 + \sum ab , where a b = 1 / 2 \sum ab = 1/2 and a 2 = 3 / 4 \sum{a}^2 = -3/4 from the given cubic equation, (using [ a ] 2 = a 2 + 2 a b [\sum a]^2 = \sum {a}^2 + 2\sum ab ) to get the answer = -1.

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