## Elementary Geometry for College Students (7th Edition)

other diagonal = $2\sqrt 63$
As we know the diagonals of a rhombus equally divides each other at 90°. So one side is the hypotenuse of the right triangle formed by the diagonals of the rhombus. now let other diagonal is 2x. then the sides of the right triangle is x and 9. by Pythagoras theorem $12^{2}=9^{2}+x^{2}$ $144=81+x^{2}$ $x^{2}=63$ $x=\sqrt 63$ other diagonal = $2\sqrt 63$