Can you find the sum of the cubes of the roots?

Algebra Level 2

a a and b b are the roots of x 2 17 x + 19 x^2-17x+19 . Find a 3 + b 3 a^3+b^3 .


The answer is 3944.

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2 solutions

By Vieta's formula, we have in this equation:

a + b = 17 a+b=17

a b = 19 ab=19

Now, we have to find a 3 + b 3 a^3+b^3 , which equals to ( a + b ) 3 3 a b ( a + b ) (a+b)^3-3ab(a+b) . Then:

a 3 + b 3 = ( 17 ) 3 3 ( 19 ) ( 17 ) a^3+b^3=(17)^3-3(19)(17)

a 3 + b 3 = 4913 969 a^3+b^3=4913-969

a 3 + b 3 = 3944 a^3+b^3=\boxed{3944}

B Sathyanarayana
Mar 13, 2014

a3+b3 = (a+b)[(a=b)2-3ab] =17[289-57]=3944

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