Let O be the midpoint of AB and connect OC.

Because △ABC is a right triangle 13? = 12? +5?

The area of a large semicircle = the sum of the areas of two small semicircles.

Area of shaded part S 1 = area of semicircular ACD-area of arcuate AC.

What is the area and sector of semi-circular ACD? The areas of OAC are equal, and the areas of arc AC are overlapping areas.

The area of the shaded part S 1 is the area of △OAC.

△OAC area = 1/2△ABC area =1/2×1/2× 5×12 =15.

A: The area of the shaded part S 1 is 15.