π×8^2 ÷2-π×5^2÷2
=π×(64-25)÷2
=3. 14×39÷2
=6 1.23(dm^2)
That is, (π times the square of 8 divided by 2-π times the square of 5 divided by 2)
On the right is the area of the square-the area of the circle:
4^2-π×(4÷2)^2
= 16-4π
3.44(cm^2)
Key point: the area formula of the circle: π r 2.
Sector area formula: π r 2-π r 2.