2.

C = π d = 3. 14× 2 = 6.28m。

S=πr？ =3. 14×2? = 12.56 m2

6.28 ÷ 0.5 =12.56 ≈12 people (13 people can't sit)

P74

1.c =πd = 3. 14×40 = 125.6cm

2. 126÷2÷3. 14≈20cm

3. 100÷( 10×2)=5

50÷( 10×2)=2.5

5×2.5= 12.5≈ 13.

4.

When the areas are equal, the circumference of a circle is the smallest and that of a rectangle is the largest.

When the perimeters are equal. The garden has the largest area and the rectangular area is the smallest.