Great Wall Of Cosine (The answer's Over 9000)!

Calculus Level 5

Given that the maximum value of:

f ( x ) = lim n ( k = 0 x cos ( k ° ) n ) f(x)=\lim _{ n\rightarrow \infty }{ \left( \sum _{ k=0 }^{ x }{ { \cos { \left( k° \right) } }^{ n } } \right) } is A A , and given that

  • x x ranges over the positive integers such that 0 x 360 0\le x\le 360

  • n n is an odd number even as it approaches \infty

  • that the sum of all the possible values of x x which would give ( f ( x ) = A ) \left(f(x)=A\right) is B B

    Find 100 ( A + B ) \left\lfloor 100(A+B) \right\rfloor


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The answer is 1647100.

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1 solution

Julian Poon
Nov 17, 2014

This is a dirt easy question when you know what it is actually asking.

When x < 1 |x|<1 lim n x n = 0 \lim _{ n\rightarrow \infty }{ { x }^{ n } } =0 . When x = 1 x=1 , lim n x n = 1 \lim _{ n\rightarrow \infty }{ { x }^{ n } } =1

Applying that to y = lim n cos ( x ) n y=\lim _{ n\rightarrow \infty }{ { \cos(x) }^{ n } } and graphing it, you'll get a beautiful horizontal line with y = 0 y=0 all the way except at points x = 2 a π x=2a\pi where y = 1 y=-1 and at points x = π ( 1 + 2 a ) x=\pi(1+2a) where y = 1 y=1 , where a a is an integer.This is assuming n n is an odd number even as it approaches \infty , which is what the question wants.

This makes the question a whole lot easier. It becomes obvious that A = 1 A=1 and this occurs when the value of x x chosen is 2 a π x < π ( 1 + 2 a ) 2a\pi \le x < \pi(1+2a) . Now, I'll convert the use of radians into degrees.

Since 0 ° x 360 ° 0° \le x \le 360° , the possible values that x x can be for f ( x ) = 1 f(x)=1 is 0 ° x < 180 ° 0° \le x < 180° and x = 360 ° x=360° . And since x x is an integer in degrees, B = ( k = 0 179 k ) + 360 = 16110 + 360 = 16470 B=\left( \sum _{ k=0 }^{ 179 }{ k } \right) +360=16110+360=16470 Therefore 100 A + B = 100 16470 + 1 = 1647100 100\left\lfloor A+B \right\rfloor = 100\left\lfloor 16470 + 1 \right\rfloor = \boxed{1647100}

You are right. Once you figure out what the question is really all about, it is pretty easy.

Mohnish Chakravarti - 6 years, 5 months ago

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