Areas of Similar Triangles

In $∆ ABC$, $XY\parallel BC$ and, $XY=\frac{1}{2}BC$. If the area of $∆AXY=10 c{m}^2$, find the area of trapezium $XYCB$.

Prove that the ratio of the area of two similar triangles is equal to the square of the ratio of their corresponding sides.