# What is the Cartesian form of ( 13 , (-9pi)/4 ) ?

Dec 26, 2015

First, find the coterminal angle for $\frac{- 9 \pi}{4}$

#### Explanation:

coterminal angle $= 4 \pi - \frac{9 \pi}{4} = \frac{7 \pi}{4}$

$\frac{7 \pi}{4}$ is in quadrant IV , so x is positive and y is negative .

$x = 13 \cos \left(\frac{7 \pi}{4}\right) = 13 \frac{\sqrt{2}}{2}$

$y = 13 \sin \left(\frac{7 \pi}{4}\right) = 13 \frac{- \sqrt{2}}{2}$

Cartesian Form $= \left(6.5 \sqrt{2} , - 6.5 \sqrt{2}\right)$

hope that helped