Take diagonal PR. It divides the rhombus into 2 triangles PQR and PTR. These 2 triangles are congruent - why? Because 2 sides PQ and TR are equal, so are PT and QR (sides of a rhombus are equal) and the side PR is common. So the corresponding angles of the 2 triangles are equal. i.e. Angle PRT = angle PRQ So PR bisects the angle R and also angle P. You can similarly prove for the other diagonal QT also. Hope you got it :)