Im trying to teach myself some geometry on my own, but im stuck since some weeks, with a couple of area problems 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 that has 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 what 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 that has 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 since some weeks, I 'll really appreaciate any help. Thanks a lot in advance.