The area formula of parallelogram is derived from the area formula of rectangle. We know that the area formula of a rectangle is equal to the product of two sides.

Derive the area of the rectangle as shown in the figure below:

In rectangular ABCD, point A and point C are regarded as high lines AE and FC respectively. According to the characteristics of parallelogram, we can know that triangle ABE is all equal to triangle FCD. Now, if the triangle FCD is translated to the left, so that point D coincides with point A, a rectangle can be formed, but the area of the rectangle remains the same during the movement.

Then, area of parallelogram = area of new rectangle = side length BC* height AE,

And high AE=sina* side AB (sine theorem in right triangle)

So parallelogram area = side BC*sina* side AB,

In other words, the area is equal to the product of two sides multiplied by the sine of the included angle.