Consecutive angles are supplementary (A + D = 180°). If one angle is right, then all angles are right. The diagonals of a parallelogram bisect each other. Each diagonal of a parallelogram separates it into two congruent triangles. A square has got 4 sides of equal length and 4 right angles (right angle = 90 degrees). Since ALL the angles in aquadrilateral add up to 360 then 360 divided by 4 must be 90.
