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.