A rhombus is a quadrilateral that has four congruent sides. Prove the following theorems about rhombi.
(a) Every rhombus is convex.
(b) The diagonals of a rhombus intersect at a point that is the midpoint of each diagonal.
(c) The diagonals of a rhombus are perpendicular.
(d) Every rhombus is a parallelogram.
