1 We can think that the total area of a circle should be a circle with an outer diameter, and the hollow part is a circle with an inner diameter. The area of a big circle minus the hollow part is the annular area. The area π of the circle is multiplied by the square of the radius, so

9 * 9 * 3.14-79-* 7 * 7 * 3.14 =100.48 cm2.

We can imagine the outer circle, the inner circle and the ring as three circles. From the relationship, their radius is the radius of the big circle minus the radius of the ring equals the radius of the small circle. But our technical area cannot be directly multiplied by 3. 14. Because the premise of our technical circle area is the square of radius, it is concluded that

(9 * 9-7 * 7) * 3.14 =100.48 cm2