Perimeter of Rectangle/Proof 2

Theorem

Let $ABCD$ be a rectangle whose side lengths are $a$ and $b$.

The perimeter of $ABCD$ is $2 a + 2 b$.

Proof

From Rectangle is Parallelogram, $ABCD$ is a parallelogram.

The result then follows from a direct application of Perimeter of Parallelogram.

$\blacksquare$