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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.