Date: Jun 12, 2008 10:15 PM
Author: Al
Subject: Area problems

Im trying to teach myself some geometry on my own, but im stuck right now with a couple of area problem of triangles. I ll really appreciate any help or advice about it. 
The formulas for a triangle are:

A=1/2 bh and A=1/2 ab Sen C

The problem says:
-Find the area of a triangle with two adyacent sides that are 5 and 4 ,and they include an angle that is 45º

I thought that the formula might be A=1/2 ab Sen C

so it would be A= 1/2 (4)(5)(sen 45) = 7.07106...

but the result on the book was = 5 square root of 2

Could I use sen of 45 in that way? if not waht is the correct procedure in order to do that problem?

There are some other similar problems that I cant understand as well:

-Find the area of a triangle with two adjacent sides that are 8 and 12, and they include an angle that is 60º.

-Find the area of a triangle if two sides are 13 and 15, and the hight over the third side is 12.

Im stuck with those ones, I 'll really appreaciate any help.

Thanks a lot in advance.