3,6.28× 2 ÷ 3.14 = 4, so the radius =2, the rectangular area =2×4=8, and the shadow area =8-6.28= 1.72.

4、2r？ = square of right angle = 12, r? =6, shadow area =3. 14×6÷2-6=3.42.

5. The triangle should be a right triangle! r？ = 16, shadow area = 3.14×16 ÷ 4-8 = 4.56.

6, small circle radius square -r? =3, the square of the diameter d-d? = 12, that is, the square area is 12.

Square of the radius of the great circle -r? =r？ +r？ =6, shadow area =3. 14×6- 12=6.84.