Chains don't always rule

Calculus Level 4

How many 1's are there in the base 3 expansion of the following expression?

d 2016 d x 2016 4 cos 3 ( x ) x = 0 \left. \frac{d^{2016}}{dx^{2016}} 4\cos^{3} (x) \right|_{x=0}


The answer is 2.

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1 solution

Isaac Buckley
Aug 12, 2015

Proof sketch

All we need to do is determine the coefficient of the x 2016 x^{2016} term in 4 cos 3 ( x ) 4\cos^3(x) , the rest isn't imporant because after taking the 201 6 t h 2016^{th} derivative and plugging in x = 0 x=0 all others terms will disappear.

We know 4 cos 3 ( x ) = cos ( 3 x ) + 3 cos ( x ) 4\cos^3(x)=\cos(3x)+3\cos(x) .

Therefore the x 2016 x^{2016} term will be: 3 2016 + 3 2016 ! x 2016 \frac{3^{2016}+3}{2016!}x^{2016}

d 2016 d x 2016 4 cos 3 x x = 0 = 3 2016 + 3 \therefore \left. \frac{d^{2016}}{dx^{2016}} 4\cos^{3} x \right|_{x=0}=3^{2016}+3

In base 3 3 this is obviously 100...010 100...010 consisting of only 2 1 s \boxed{2}\, 1's

Pretty much.

Jake Lai - 5 years, 10 months ago

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