Question

prove that the diagonal of a rhombus are intersect at 90 degree​

Answer 1

In any rhombus, the diagonals (lines linking opposite corners) bisect each other at right angles (90°).

That is, each diagonal cuts the other into two equal parts, and the angle where they cross is always 90 degrees. In the figure above drag any vertex to reshape the rhombus and convince your self this is so.