# Find $\sin \theta$ and $\cos \theta$ given $\tan 2\theta$

Can you guys help with verifying my work for this problem. My answers don't match the given answers.

Given $\tan 2\theta = -\dfrac{-24}{7}$, where $\theta$ is an acute angle, find $\sin \theta$ and $\cos \theta$

I used the identity, $\tan 2\theta = \dfrac{2\tan \theta}{1 - tan^2 \theta}$ to try and get an equation in $\tan \theta$.

\begin{align} -\dfrac{24}{7} &= \dfrac{2\tan \theta}{1 - \tan^2 \theta} \\ -24 + 24\tan^2 \theta &= 14 \tan \theta \\ 24tan^2 \theta - 14\tan \theta - 24 &= 0 \\ 12tan^2 \theta - 7\tan \theta - 12 &= 0 \\ \end{align}

Solving this quadratic I got, $$\tan \theta = \dfrac{3}{2} \text{ or } \tan \theta = -\dfrac{3}{4}$$

$$\therefore \sin \theta = \pm \dfrac{3}{\sqrt{13}} \text{ and } \cos \theta = \pm \dfrac{2}{\sqrt{13}}$$

Or,

$$\therefore \sin \theta = \pm \dfrac{3}{5} \text{ and } \cos \theta = \mp \dfrac{4}{5}$$

The given answer is,

$$\sin \theta = \dfrac{4}{5} \text{ and } \cos \theta = \dfrac{3}{5}$$

I thought I needed to discard the negative solution assuming $\theta$ is acute. But they haven't indicated a quadrant. Do I assume the quadrant is I only? What am i missing? Thanks again for your help.

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You didn't solve the quadratic for $\tan\theta$ correctly. – Gerry Myerson Jul 5 '11 at 7:23
The condition given is $\theta$ is an acute angle. – Mark Bennet Jul 5 '11 at 7:24
If $\theta$ is an acute angle then you only want solutions where $\tan \theta$, $\sin \theta$, and $\cos \theta$ are all positive. – Henry Jul 5 '11 at 7:30
I guess it is to some extent a matter of convention, whether you call an angle in the range $(-\pi/2,0)$ acute or not. If you draw it, the angle sure looks acute. Whenever I give my students a problem like this, I specify the quadrant (unless I want them to find all the possible solutions). I don't know, if there is a standard for this in the English speaking regions of the world. – Jyrki Lahtonen Jul 5 '11 at 7:38
Thanks everyone, found the error. $\tan \theta = \dfrac{4}{3}$ and hence the given values of $\sin$ and $\cos$ follow. Also I am assuming that $\theta$ is acute in most similar problems implicitly implies Quadrant I, which explains why the given answers are only +ve. – mathguy80 Jul 5 '11 at 7:46

## 2 Answers

At Chandru's request:

1. The quadratic $12z^2-7z-12$ factors as $(3z-4)(4z+3)$ so we should get $\tan\,\theta=4/3$ and $\tan\,\theta=-3/4$.

2. "Acute angle" means "angle between 0 and $\pi/2$" means 1st quadrant.

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Thanks a lot :) – user9413 Jul 5 '11 at 17:15

Given $tan(2\theta)= -\frac{24}{7}$ From the relation between $sin(\theta)$, $cos(\theta)$ and $tan(\theta)$, we get $$\frac{sin(2\theta)}{cos(2\theta)}= -\frac{24}{7} \implies sin(2\theta)= -\frac{24}{7} cos(2\theta)$$and $$sin(2\theta)^2 + cos(2\theta)^2=1$$ $$cos(2\theta) = \pm \frac{7}{25} \implies 2 cos^2(\theta)-1 = \pm \frac{7}{25}$$ Case 1: Rational number on the right is positive, $$cos^2(\theta)=\frac{16}{25} \implies cos(\theta) = \pm \frac{4}{5}$$ Solution to case 1:
$$cos(\theta)=\frac{4}{5}$$$$sin(\theta)=\frac{3}{5}.$$ Both sine and cosine functions are positive, for $\theta$ being acute.
Case 2:Rational number on the right is negative $$cos^2(\theta)=\frac{9}{25} \implies cos(\theta) = \pm \frac{3}{5}$$Solution to case 2:
$$cos(\theta)=\frac{3}{5}$$$$sin(\theta)=\frac{4}{5}.$$ Both sine and cosine functions are positive, for $\theta$ being acute.

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