a parallelogram is not in general a rectangle

(drawn with interior angle 60°, width 4, and height 3)

independent edge lengths | a | b width | b height | a sin(θ) diagonal lengths | sqrt(a^2 + 2 a b cos(θ) + b^2) | sqrt(a^2 - 2 a b cos(θ) + b^2) area | a b sin(θ) perimeter | 2 (a + b) independent interior angles | (θ rad | (π - θ) rad)≈(θ rad | (3.14159 - θ) rad) interior angle sum | 360° = 2 π rad≈6.283 rad (assuming center (x_0, y_0), interior angle θ rad, and edge lengths a, b)