According to the vertical diameter theorem:

R & ampsup2r & ampsup2= 3 & ampsup2=9

Semicircular area = π r&; sup2/2

Area of small semicircle = π r&; sup2/2

∴ Shadow area s = (π r&; sup2/2)-(πr & amp； sup2/2)=(π/2)(R & amp； sup2r & ampsup2)=9π/2