Rewrite each expression as the sine, cosine, or tangent of an angle.  

1. $\sin\left(\frac{3\pi}{2}\right)\cos\left(\frac{\pi}{7}\right)-\cos\left(\frac{3\pi}{2}\right)\sin\left(\frac{\pi}{7}\right)$ 

   Hint (a):

   Look at the angle difference formula for sine.

2. $\cos\left(\frac{\pi}{3}\right)\cos\left(\frac{3\pi}{10}\right)-\sin\left(\frac{\pi}{3}\right)\sin\left(\frac{3\pi}{10}\right)$ 

   Hint (b):

   Look at the angle sum formula for cosine.

3. $\frac { \operatorname { tan } ( 203 ^ { \circ } ) - \operatorname { tan } ( 6 ^ { \circ } ) } { 1 + \operatorname { tan } ( 203 ^ { \circ } ) \operatorname { tan } ( 6 ^ { \circ } ) }$ 

   Hint (c):

   Look at the angle difference formula for tangent.