IS. 1 In figure $1, \triangle A B C$ is an acute triangle, $A B=15, A C=13$, and its altitude $A D=12$. If the area of the $\triangle A B C$ is $P$, find the value of $P$.
$\begin{array}{l}B D=\sqrt{15^{2}-12^{2}}=9 \\ C D=\sqrt{13^{2}-12^{2}}=5 \\ P=\text { area of } \Delta=\frac{1}{2}(9+5) \times 12=84\end{array}$