1. Multiply half the circumference by half the diameter, or multiply the whole circumference by the whole diameter, and take ¼ of the product.
2. Or, square the diameter, and multiply that square by ·7854 for the area.
3. Or, square the circumference, and multiply that square by 0·7958.
Example 1.—Required the area of a circle, whose circumference is 55·548 inches, and its diameter 18 inches.
55·548 2 = 27·774 half circumference.
18 2 = 9 half diameter.
27·774 × 9 = 249·966, square inches. Area required.
Example 2.—Required the area of a circle whose diameter is 12 feet.
12 × 12 = 144, square of the diameter.
·7854 × 144 = 113·0976 square feet. Area required.
Example 3.—Required the area of a circle, whose circumference is 22 feet.
22 × 22 = 484.
484 × ·07958 = 38·51672 square feet. Area required.