# Difference between revisions of "2013 AIME I Problems/Problem 14"

## Problem 14

For $\pi \le \theta < 2\pi$, let

\begin{align*} (Error compiling LaTeX. ! Package amsmath Error: \begin{align*} allowed only in paragraph mode.) $P &= \frac12\cos\theta - \frac14\sin 2\theta - \frac18\cos 3\theta + \frac{1}{16}\sin 4\theta + \frac{1}{32} \cos 5\theta - \frac{1}{64} \sin 6\theta - \frac{1}{128} \cos 7\theta + \cdots$ (Error compiling LaTeX. ! Misplaced alignment tab character &.) \end{align*} (Error compiling LaTeX. ! Misplaced \cr.)

and

\begin{align*} (Error compiling LaTeX. ! Package amsmath Error: \begin{align*} allowed only in paragraph mode.) $Q &= 1 - \frac12\sin\theta -\frac14\cos 2\theta + \frac18 \sin 3\theta + \frac{1}{16}\cos 4\theta - \frac{1}{32}\sin 5\theta - \frac{1}{64}\cos 6\theta +\frac{1}{128}\sin 7\theta + \cdots$ (Error compiling LaTeX. ! Misplaced alignment tab character &.) \end{align*} (Error compiling LaTeX. ! Misplaced \cr.)

so that $\frac{P}{Q} = \frac{2\sqrt2}{7}$. Then $\sin\theta = -\frac{m}{n}$ where $m$ and $n$ are relatively prime positive integers. Find $m+n$.

## Solution

### Solution 1

\begin{align*} (Error compiling LaTeX. ! Package amsmath Error: \begin{align*} allowed only in paragraph mode.) $P \sin\theta\ + Q \cos\theta\ = \cos\theta\ - \frac{1}{2}\ P$ \end{align*} (Error compiling LaTeX. ! Misplaced \cr.) and \begin{align*} (Error compiling LaTeX. ! Package amsmath Error: \begin{align*} allowed only in paragraph mode.) $P \cos\theta\ + Q \sin\theta\ = -2(Q-1)$ \end{align*} (Error compiling LaTeX. ! Misplaced \cr.)

Solving for P, Q we have

$\frac{P}{Q} = \frac{\cos\theta }{2 + \sin\theta } = \frac{2\sqrt2}{7}$

Square both side, and use polynomial rational root theorem to solve $\sin\theta$

$\sin\theta = -\frac{17}{19}$

The answer is $\boxed{036}$

### Solution 2

\begin{align*} (Error compiling LaTeX. ! Package amsmath Error: \begin{align*} allowed only in paragraph mode.) Use sum to product formulas to rewrite $P$ and $Q$ \end{align*} (Error compiling LaTeX. ! Misplaced \cr.)

$P \sin\theta\ + Q \cos\theta\ = \cos \theta\ - \frac{1}{4}\cos \theta + \frac{1}{8}\sin 2\theta + \frac{1}{16}\cos 3\theta - \frac{1}{32}\sin 4\theta + ...$

\begin{align*} (Error compiling LaTeX. ! Package amsmath Error: \begin{align*} allowed only in paragraph mode.) Therefore, $P \sin \theta - Q \cos \theta = -2P$ \end{align*} (Error compiling LaTeX. ! Misplaced \cr.)

\begin{align*} (Error compiling LaTeX. ! Package amsmath Error: \begin{align*} allowed only in paragraph mode.) Using $\frac{P}{Q} = \frac{2\sqrt2}{7}$, $Q = \frac{7}{2\sqrt2} P$ \end{align*} (Error compiling LaTeX. ! Misplaced \cr.)

\begin{align*} (Error compiling LaTeX. ! Package amsmath Error: \begin{align*} allowed only in paragraph mode.) Plug in to the previous equation and cancel out the "P" terms to get: $\sin\theta - \frac{7}{2\sqrt2} \cos\theta = -2$ \end{align*} (Error compiling LaTeX. ! Misplaced \cr.)

Then use the pythagorean identity to solve for $\sin\theta$, $\sin\theta = -\frac{17}{19} \implies \boxed{036}$

### Solution 3

Note that $$e^{i\theta}=\cos(\theta)+i\sin(\theta)$$

Thus, the following identities follow immediately: $$ie^{i\theta}=i(\cos(\theta)+i\sin(\theta))=-\sin(\theta)+i\cos(\theta)$$ $$i^2 e^{i\theta}=-e^{i\theta}=-\cos(\theta)-i\sin(\theta)$$ $$i^3 e^{i\theta}=\sin(\theta)-i\cos(\theta)$$

Consider, now, the sum $Q+iP$. It follows fairly immediately that:

$$Q+iP=1+\left(\frac{i}{2}\right)^1e^{i\theta}+\left(\frac{i}{2}\right)^2e^{2i\theta}+\ldots=\frac{1}{1-\frac{i}{2}e^{i\theta}}=\frac{2}{2-ie^{i\theta}}$$ $$Q+iP=\frac{2}{2-ie^{i\theta}}=\frac{2}{2-(-\sin(\theta)+i\cos(\theta))}=\frac{2}{(2+\sin(\theta))-i\cos(\theta)}$$

This follows straight from the geometric series formula and simple simplification. We can now multiply the denominator by it's complex conjugate to find:

$$Q+iP=\frac{2}{(2+\sin(\theta))-i\cos(\theta)}\left(\frac{(2+\sin(\theta))+i\cos(\theta)}{(2+\sin(\theta))+i\cos(\theta)}\right)$$ $$Q+iP=\frac{2((2+\sin(\theta))+i\cos(\theta))}{(2+\sin(\theta))^2+\cos^2(\theta)}$$

Comparing real and imaginary parts, we find: $$\frac{P}{Q}=\frac{\cos(\theta)}{2+\sin(\theta)}=\frac{2\sqrt{2}}{7}$$

Squaring this equation and letting $\sin^2(\theta)=x$:

$\frac{P^2}{Q^2}=\frac{\cos^2(\theta)}{4+4\sin(\theta)+\sin^2(\theta)}=\frac{1-x^2}{4+4x+x^2}=\frac{8}{49}$

Clearing denominators and solving for $x$ gives sine as $x=-\frac{17}{19}$.

$017+019=\boxed{036}$