When is a rhombus a rectagle?

Answer 1

A rhombus is a rectangle when one of its angles is a right angle.

(Actually, a rectangle has 4 right angles. But you only have to prove that your

rhombus has one of them for sure, because then it turns out that it must have

four of them.)

(Also, before you get all upset because a rhombus with right angles is actually

a square, we're aware of that too. Technically, a square is a special kind of

parallelogram, rhombus and rectangle too.)