Continue the computation we did today.  I computed a matrix A
such that A times v is the same as rotating v by alpha degrees.

Now, lets take u = e1.
Then rotate u by beta degrees, and call the result v.
Then multiply A by v.

The resulting vector must be:
 the vector you get by rotating e1 by alpha+beta degrees.


Notice that by doing this computation, you re-discovered
the sin and cosine angle-addition formulas!

	sin(alpha + beta) = ....

	cos(alpha + beta) = ...

(where you can fill in the dots by the computation A times v).
This way, if you forget the angle-addition formulas, you now
know how to recompute them.




Section 1.9 exercises:

Make sure you can do Ex 1-10.

Turn in Ex 12, Ex 14, Ex 18.

Notice that Ex 17-20 are just like Ex 15,16
so you may want to look at those first before you do Ex 18.

Read Ex 23-35 in order to test if you understand the concepts.