Calculate the area of a circle whose diameter is 1) 18 cm; 2) 3.6 meters

The area of a circle is determined by the formula S = pi * R ^ (2), where pi = 3.14. There are 2 ways to calculate the area of a circle by knowing the diameter.

Method 1: knowing that the radius is half the diameter, divide the diameter by 2.
That is, a) 18/2 = 9 cm.This is the radius of the first circle, we substitute it into the formula:
S = 3.14 * 9 ^ (2) = 254.34 square centimeters.
b) 3.6 / 2 = 1.8 m. This is the radius of the second circle. We substitute:
S = 3.14 * 1.8 ^ (2) = 10.17 sq. m.

Method 2: it is necessary to derive the formula for the area of a circle, which will include the diameter.
S = pi * (D / 2) ^ (2) = pi * D ^ (2) / 4.