An isosceles trapezoid (called an isosceles trapezium by the British; Bronshtein and Semendyayev 1997, p. 174) is trapezoid in which the base angles are equal and therefore the left and right side lengths are also equal.
From the Pythagorean theorem,
so
An isosceles trapezoid has perimeter
and diagonal lengths
More things to try:
Weisstein, Eric W. "Isosceles Trapezoid." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/IsoscelesTrapezoid.html