21. A semicircle of radius 3 units is drawn on one edge of a right-angled triangle, and a semicircle of radius 4 units is drawn on another edge. The semicircles intersect on the hypotenuse of the triangle, as shown.
What is the shaded area, in square units?

A $\frac{25 \pi}{2}-24$
B 12
C $\frac{25 \pi}{2}-6$
D $25 \pi-24$
E 24
21. A As the semicircles intersect on the hypotenuse of the triangle, the shaded area, in square units, is the sum of the areas of the two semicircles minus the area of the right-angled triangle, that is $\frac{1}{2} \pi \times 3^{2}+\frac{1}{2} \pi \times 4^{2}-\frac{1}{2} \times 6 \times 8=\frac{25 \pi}{2}-24$.