CPM Homework Help : PC3 Problem 8-114

Evaluate each of the following trigonometric expressions.

$\cos\left(\frac{2\pi}{3}+\frac{3\pi}{4}\right)$
Step (a):
$\cos\left(\frac{2\pi}{3}\right)\cos\left(\frac{3\pi}{4}\right)-\sin\left(\frac{2\pi}{3}\right)\sin\left(\frac{3\pi}{4}\right)$

$\sin\left(\frac{7\pi}{6}-\frac{\pi}{4}\right)$
Step (b):
$\sin\left(\frac{7\pi}{6}\right)\cos\left(\frac{\pi}{4}\right)-\cos\left(\frac{7\pi}{6}\right)\sin\left(\frac{\pi}{4}\right)$