Answer:
So for our trapezium, the top side length is a, the bottom side length is b and the height is h.
So assuming b > a, b - a will give the length of the base of the two triangles. So you now have a rectangle, a by h (and the area ah) and two triangle, each with a base of (b-a)/2 triangle and height h. The area of the two triangles is just (b-a)/2 * h or h(b-a)/2.
The total area is h(b-a)/2 + ah = h(b-a)/2 + 2ah/2 = h(b-a+2a)/2 = h(b+a)/2
Step-by-step explanation:
Look at the picture
