EXPLANATION
The area of a rectangle is as follows:
[tex]\text{Area}_{\text{rectangle}}=\text{base }\cdot\text{ height}[/tex]The area of a triangle:
[tex]\text{Area}_{triangle}=\frac{1}{2}\text{base}\cdot\text{height}[/tex]So the Area of a triangle is a half the area of a rectangle:
We can decompose the irregular or nonstandard shape as the sum of multiply triangles.