Day 14: Christmas is the Solution

Calculus Level 3

C = 0 3 π [ 1 cos 2 x 12 1 sin 2 x 12 + 1 ] d x C = \int_0^{3\pi} \bigg [ \frac{1 - \cos^2{\frac{x}{12}}} {1 - \sin^2{\frac{x}{12}}} + 1 \bigg ] \, \mathrm{d}x If the value of 2 + C 144 2 + \frac{C}{144} is in the form a / b a/b where a a and b b are positive coprime integers and enter a + b a+b as your answer.

This problem is part of the set Advent Calendar 2014 .


The answer is 37.

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2 solutions

Discussions for this problem are now closed

Samuel Li
Dec 16, 2014

Anti-solution: We notice that the title is "Christmas is the Solution". Being too lazy to actually solve the problem, we recall that Christmas is on 12/25. The question asks for a+b, so we assume that {a, b} = {12, 25}. Adding results in the correct answer of 37, by chance.

Good solution: Using cos 2 x + sin 2 x = 1 \cos^2{x}+\sin^2{x}=1 , the fraction simplifies down to sin 2 x / 12 cos 2 x / 12 = tan 2 x / 12 \frac{\sin^2{x/12}}{\cos^2{x/12}}=\tan^2{x/12} . Using tan 2 x + 1 = s e c 2 x \tan^2{x}+1=sec^2{x} , we are left with the integral of sec 2 x / 12 \sec^2{x/12} , which evaluates to 12 tan x / 12 12\tan{x/12} . Evaluating from 0 to 3 π 3\pi results in the value 12. Substituting into the given format gives the fraction 25 12 \frac{25}{12} , so a + b = 37 a+b=37 .

anti solution - hats off

U Z - 6 years, 5 months ago
Michael Ng
Dec 13, 2014

A bit of simplifying shows that C C is the integral of sec 2 x 12 \sec^2{\frac{x}{12}} which gives: [ 12 tan x 12 ] 0 3 π = 12 \left[ 12\tan{\frac{x}{12}} \right]_0^{3\pi} = 12 Therefore 2 + 12 144 = 25 / 12 2 + \frac{12}{144} = 25/12 and so the final answer is 37 \boxed{37} .

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