For a ring, the sum of the circumference of the inner ring and the outer ring is 157cm. The difference between the circumference of the inner ring and the outer ring is 31.4cm, and the area of the ring is larger

For a ring, the sum of the circumference of the inner ring and the outer ring is 157cm. The difference between the circumference of the inner ring and the outer ring is 31.4cm, and the area of the ring is larger

Circumference of inner circle
(157-31.4) △ 2 = 62.8 (CM)
Circumference of outer circle
157-62.8 = 94.2 (CM)
Radius of inner circle
62.8 ﹣ 3.14 ﹣ 2 = 10 (CM)
Radius of outer circle
94.2 ﹣ 3.14 ﹣ 2 = 15 (CM)
What is the area of the ring
15 × 15 × 3.14-10 × 10 × 3.14 = 392.5 (square centimeter)

Two concentric circles, if the circumference of the small circle is 20% less than that of the big circle, if the area of the ring is 7.2 square centimeter, the area of the big circle is () square centimeter

Two concentric circles, if the circumference of the small circle is 20% less than that of the big circle, if the area of the ring is 7.2 square centimeter, the area of the big circle is (20) square centimeter
7.2÷【1-（1-20%）×（1-20%）】
=7.2÷【1-0.64】
=7.2÷0.36
=20

The perimeter of the outer circle of a bad type, the diameter of the inner circle is 4cm, and its annular area is

Let l be the circumference of the outer circle
Outer circle area s = L / 4 π
Area of inner circle s' = 4 π
The ring area is S-S'