Calculate the area of a circle whose radius is 5 cm 2. Find the diameter of a circle whose area is 81п cm2.

1. Let’s find the area of a circle by the formula S = ттr ^ 2, where тт = 3.14, r is the radius:

S = 3.14 * 5 ^ 2 = 3.14 * 25 = 78.5 cm2.

2. Knowing the area of a circle, we can find out its radius:

S = ттr ^ 2 = 81тт = 9 ^ 2тт,

r ^ 2 = 9 ^ 2,

r = 9.

Now we calculate the diameter of the circle d = 2r = 2 * 9 = 18 cm.