Area of a circle

Area A = π x r²

with π (number pi) approximately equal to 3.14
Radius = AO = OB = r
Diameter = AB = 2 r

Example

Take a circle with radius r = 5 cm
Area A of the circle = π x r² ≈ 3.14 x 5² ≈ 78.5 cm²

Calculate the area of a circle

Radius
Area of the circle