Area by Subtracting

Sometimes it is easier to start big and take away. A rectangle with a corner cut off is the whole rectangle minus the corner triangle. Area of the parts works both ways: if the whole and one piece are known, the other piece is the difference.

Here the rectangle is 10 by 8, area 80. The corner triangle ECF has a right angle at C and legs 4 and 6, so its area is $\frac{1}{2}\cdot 4\cdot 6 = 12$. The piece that is left has area $80-12 = 68$. In a right triangle the two legs are a base and its height.