How is the trapezoidal area formula derived?

How is the trapezoidal area formula derived?

Let the upper bottom be a, the lower bottom be B, and the height be H
Connecting any trapezoid diagonal, the trapezoid is divided into two triangles of the same height
S trapezoid = 1 / 2 * a * H + 1 / 2 * b * H = 1 / 2 * (a + b) * h