The shaded part can be regarded as a big semicircle+a small semicircle.

Large semicircle: half a circle (that is, half a circle) =6? ×π÷2

Small semicircle: half a circle (that is, half a circle) =3? ×π÷2

A big semicircle+a small semicircle =6? × pi ÷2+3？ xπ÷2 =(6？ +3? )×π÷2

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mathematical formula

(6? +3? )×π÷2

=(36+9)×π÷2

=22.5 times π

What is the value of pi? See for yourself. (It should be 3. 14)