# converting cos to sin and tan in specific quadrants

I'm having issues understanding as to how to go about doing this. I cant seem to figure out how to find the values of sin and tan in terms of the given cos value in the 3rd quadrant. Thanks with any and all help.

$\cos[\theta] = \frac{-4}{5}$ and theta is in the 3rd quadrant, find the exact values of (i) $\sin[\theta]$ (ii) $\tan[\theta]$

• Well, what do you know about the sign of the sine and tangent functions when the angle is in the third quadrant? If you use this and Pythagoras' Theorem, you should be able to get the answer. – quasicoherent_drunk Aug 22 '15 at 2:33
• Recall that $cos(\theta) = \frac{\text{adjacent}}{\text{hypotenuse}}$ – Paddling Ghost Aug 22 '15 at 2:33
• Remember that $\cos\theta$ is just the $x$-coordinate of a point at angle $\theta$ on the unit circle. Knowing that $\cos\theta = -\frac45$ and that $\theta$ is in the third quadrant, you should be able to make a rough sketch of that point. – David K Aug 22 '15 at 2:43