
geometry problem
I know I'm going to seem like a real stoop, but this has me baffled because there are NO examples like it in my textbook.
Here's the problem: Find the area of a rectangle with length 4 and diagonal 5.
I know to find the area of a rectangle is A=bh, but don't know how to figure in the diagonal part!(Headbang)
Any help is appreciated!!!!

You must use Pythagore theorem that is $\displaystyle c^2 = a^2 + b^2 $ c is the diagonal.
For more information on Pythagore theorem go on wikipedia.
$\displaystyle \sqrt{c^2  a^2} = b = 3 $
And you can do the rest as you told me.