Let a , b , and c be the roots of the equation 2 x 3 − x 2 + x + 3 = 0 . Find the value of the expression below.
a − b a 3 − b 3 + b − c b 3 − c 3 + c − a c 3 − a 3
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The answer is -1, not 1. It's 2 ( a 2 + b 2 + c 2 ) + a b + a c + b c .
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Sorry I had typo mistake
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@Nihar Mahajan , A typo is a mistake. There is nothing Known as Typo Mistake :3
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@Mehul Arora – Well , if I had written this solution on paper with my "hand" such "typo" mistake would never happen.
BTW @Pi Han Goh Can you please help in this note . Thanks,
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I'm not quite good at writing proofs in geometry. Lemme reshare that!
Simplify the given expression to get 2 ( a 2 + b 2 + c 2 ) + a b + b c + a c . Since a 2 + b 2 + c 2 = ( a + b + c ) 2 − 2 ( a b + b c + a c ) , we have
2 ( a 2 + b 2 + c 2 ) + a b + b c + a c
= 2 [ ( a + b + c ) 2 − 2 ( a b + b c + a c ) ] + a b + b c + a c
= 2 ( a + b + c ) 2 − 4 ( a b + b c + a c ) + a b + b c + a c
= 2 ( a + b + c ) 2 − 3 ( a b + b c + a c ) . Substitute in the values and we are done.
Cancelling out from numerator and denominator common factors, the expression reduces to 2 ∑ a 2 + ∑ a b , where ∑ a b = 1 / 2 and ∑ a 2 = − 3 / 4 from the given cubic equation, (using [ ∑ a ] 2 = ∑ a 2 + 2 ∑ a b ) to get the answer = -1.
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Using Vieta's formula ,
a + b + c = 2 1 a b + b c + a c = 2 1
Using Newton's identity ,
a 2 + b 2 + c 2 = ( a + b + c ) 2 − 2 ( a b + b c + a c ) = ( 2 1 ) 2 − 2 ( 2 1 ) = 4 − 3
By using identity x 3 − y 3 = ( x − y ) ( x 2 + x y + y 2 ) ,the terms like ( a − b ) , ( b − c ) , ( c − a ) get cancelled ( a = b = 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 ( 4 − 3 ) + 2 1 = − 1