Circle : A circle is the locus of a point which moves in a plane in such a way that its distance from a fixed point always remains same. The fixed point is called the centre and the given constant distance is known as the radius of the circle.
If r is the radius of a circle, then
(i) Circumference = 2πr or πd ,
where d = 2r is the diameter of the circle.
(iii) Area of semi circle
(iv) Area of quadrant of a circle