Geometry Problem - 2

$ABCD$ is a cyclic quadrilateral. Diagonals $AC$ and $BD$ meet at $E$ . If $AE=2, BE=5,CE=10,DE=4$ and $BC=\dfrac{15}{2}$ , find $AB$ . If your answer can be expressed as $\dfrac{1}{2}\sqrt{a}$ , give the value of $a$ .