The area of the shaded section is ( ¼·Area of circle with radius = 20 cm ) - ( ½·Area of circle with diameter = 20 cm)
The formula for the area of a circle is: A = pi·radius2
First, find the area of the circle with radius = 20 cm: Area = pi·202
then, find one-fourth of this value: Area = ¼·pi·202 <--- Answer #1
Second, find the area of the circle with diameter = 20 cm:
radius = ½·diameter = ½·20 cm = 10 cm
then, find the area of this circle: Area = pi·102
then, find one-half of this value: Area = ½·pi·102 <--- Answer #2
Now, subtract Answer #2 from Answer #1 to get your final answer.