State the exact value of each of the following trigonometric expressions without using a calculator or referring to your unit circle. 

1. $\cos\left(\frac{-5\pi}{6}\right)$ 

   Step (a):

   $-\frac{5\pi}{6}=-\frac{5\pi}{6}+2\pi=\frac{7\pi}{6}$

   What is $\cos\Big(\frac{7\pi}{6}\Big)$?

2. $\tan\left(\frac{31\pi}{4}\right)$ 

   Step (b):

   $\frac{31\pi}{4}=\frac{31\pi}{4}-6\pi=\frac{7\pi}{4}$