This example demonstrates how the area of a parallelogram formula can be derived using the formulas for the area of a rectangle.
Start by observing that a parallelogram’s area can be divided into a rectangle and two triangles. This is shown in the illustration below.
Then observe that the triangles can be rearranged by sliding one triangle underneath the other triangle
Finally, observe that the shape we are left with is the shape of a rectangle and from the area of a rectangle formula, we can conclude that the area of a parallelogram is equal to the base multiplied by the height.