Answer
$\frac{11AD-A^2D-12D}{24*(A+12)}$
Work Step by Step
Young's Rule: $DA/(A+12)$
Cowling's Rule: $D(A+1)/24$
$\frac{DA}{(A+12)}-\frac{D(A+1)}{24}$
$\frac{24*DA}{24*(A+12)}-\frac{D(A+1)(A+12)}{24(A+12)}$
$\frac{24DA}{24*(A+12)}-\frac{D(A+1)(A+12)}{24(A+12)}$
$24DA-D(A+1)(A+12)$
$24DA-(D(A^2+13A+12))$
$24DA-A^2D-13AD-12D$
$11AD-A^2D-12D$
$\frac{11AD-A^2D-12D}{24*(A+12)}$
