1.   Circumference of a circle = 2 $πr$.

2.   Area of a circle = $πr^2$.
 
3.   Length of an arc of a sector of a circle with radius r and angle with degree measure θ is
                         $$ ={ θ/360}× 2 πr$$                

4.  Area of a sector of a circle with radius r and angle with degree measure θ is
						$$ ={ θ/360}× πr^2$$ 

5.   Area of segment of a circle = Area of the corresponding sector – Area of the corresponding triangle.