Isoceles trapezoid: base angles equal

Dependencies: Parallelogram tests, Parallelogram properties (opposite sides equal, diagonals bisect each other), Base angles of an isoceles triangle are equal, Parallel ⇒ corresponding angles equal (converse).

Statement

Let $ABCD$ be an isosceles trapezoid:

where $AD$ is the upper base (shorter), $BC$ is the lower base (longer), and $AB$, $CD$ are the two legs. Then the two base angles on the same base are equal: