Twisted Binomial

Algebra Level 4

What is the largest positive integer less than ( 6 + 5 ) 6 . (\sqrt{6} + \sqrt{5})^{6} .


The answer is 10581.

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1 solution

Danish Ahmed
Dec 29, 2015

Let x = 6 , y = 5 x=\sqrt{6},y=\sqrt{5} .

Thus ( x + y ) 6 + ( x y ) 6 = 2 ( x 2 + y 2 ) ( x 4 + 14 x 2 y 2 + y 4 ) = 10582. (x+y)^6+(x-y)^6=2\left(x^2+y^2\right)\left(x^4+14x^2y^2+y^4\right)=10582.

But 0 < ( x y ) 6 < 1 0<(x-y)^6<1 , so ( 6 + 5 ) 6 = 10581 \boxed{\left\lfloor\left(\sqrt{6}+\sqrt{5}\right)^6\right\rfloor=10581}

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