A circle is a simple closed curve. It is the locus of points which is equidistant from the centre of the circle.
Here O is the centre of the circle and r is the radius of the circle.

Area of circle = A=πr^{2}

Circumference of circle = C = 2πr

Diameter of Circle = D = 2r
Ex: The diameter of a circle is 6cm. Find the Area and circumference of a circle.
Answer: D = 2r
6 = 2r
r = 3cm
Area of circle = πr^{2} = π (3)^{2} = 9π cm^{2}
Circumference of circle = 2πr = 2π(3) = 6π cm
Ex: Circumference of a circle is 44cm. Find the Area of the circle.
Answer: Circumference of a circle = 2πr = 44 cm
r = ^{44}⁄_{2π}
r = 7cm
Area of a circle = πr² = π (7)² = 49π cm²