Something interesting in cubes

Algebra Level 4

8 x 3 12 x 2 6 x 1 = 0 \large 8x^{3}-12x^{2}-6x-1=0

If the value of a real root that satisfy the equation above can be expressed as

a 3 + b 3 + 1 c \large \dfrac{\sqrt[3]{a}+\sqrt[3]{b}+1}{c}

where a , b a,b and c c are positive integers, find the value of a + b + c a+b+c .


The answer is 8.

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1 solution

Go through the simplification below, 8 x 3 12 x 2 6 x 1 = 0 8x^{3}-12x^{2}-6x-1=0 16 x 3 ( 8 x 3 + 12 x 2 + 6 x + 1 ) = 0 16x^{3}-\left ( 8x^{3}+12x^{2} +6x+1\right )=0 16 x 3 ( 2 x + 1 ) 3 = 0 16x^{3}-\left ( 2x+1 \right )^{3}=0 16 x 3 = ( 2 x + 1 ) 3 16x^{3}=\left ( 2x+1 \right )^{3} In considering in this way we may be able to find the real root with a , b , c Z + a,b,c \in \mathbb{Z^{+}} satisfying given conditions, 16 3 x = 2 x + 1 \sqrt[3]{16}x=2x+1 x ( 16 3 2 ) = 1 x\left ( \sqrt[3]{16}-2\right )=1 x = 1 16 3 2 x=\frac{1}{\sqrt[3]{16}-2} x = 1 6 2 3 + 2 16 3 + 4 8 x=\frac{\sqrt[3]{16^{2}}+2\sqrt[3]{16}+4}{8} x = 4 4 3 + 4 2 3 + 4 8 x=\frac{4\sqrt[3]{4}+4\sqrt[3]{2}+4}{8} x = 4 3 + 2 3 + 1 2 a + b + c = 8 x=\frac{\sqrt[3]{4}+\sqrt[3]{2}+1}{2}\Rightarrow a+b+c=8 \therefore It concludes that the relavant answer is 8

Nice problem. There was a similar problem on the 2013 AIME I that I was unable to solve.

Alex Li - 5 years, 11 months ago

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Thanks.. Yes, there was a similar one in 2013 AIME I.

A Former Brilliant Member - 5 years, 11 months ago

Awesome solution !! :P

Rohit Udaiwal - 5 years, 8 months ago

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