A quadrilateral has the vertices A = (0,0), B = (2,4), C = (0,5), D = (-2,1). Prove that ABCD is a rectangle. so they are so they are parallel. Therefore, we Since the slopes of sides AB and BC have a product , meaning angle ABC is a right angle. Since ABCD is Sides AB and CD both have slopes of and AD both have slopes of know ABCD is a of -1, they are a parallelogram with at least one right angle, it must also be a square rhombus rectangle triangle . Sides BC