Limit of triangle in a circle

$\triangle ABC$ is an isosceles triangle inscribed in a circle of radius $10$ . If $AB=AC$ , then evaluate $\displaystyle \lim_{h \to 0} \frac {\phi}{P^{3}}$ , where $h$ is altitude from point $A$ to $BC$ , $\phi$ is area of triangle and $P$ is perimeter of triangle. If the answer is $\dfrac {X}{Y}$ , then find the value of $XY$