How do you solve the area of a trapezoid using diagonals The height of a trapezoid is $10$ cm. The lengths of the two diagonals of the trapezoid are $30$ cm and $50$ cm. Calculate the area of the trapezoid. 
On the homework I solved this using $${D_1D_2\over 2}$$ and my teacher marked me wrong. So I don't know what I did wrong. Please help. I know I can only use the formula if the diagonals are $90$ degrees. But how do I check that ? 

To help you think about the solution, draw two parallel lines 10 cm apart.  The bottom will contain the B1 and the top will contain the B2.  Draw the two diagonals at the proper lengths from the base to the top line making sure they cross.  Now imagine that you slide one of the diagonals along the bottom and top lines.  Notice that (B2+B2)/2 does not change (i.e one stretches and one shrinks).  So no matter where the diagonals cross you have the same area.  Now slide them apart until the intersection point reaches the top line (i.e. B2 = 0).  Now you have  triangle with the same area as the trapezoid.  You know the height and two sides of the triangle, a little geometry and you can compute the base and your're there.
