a) If is the radius of a circle then:
- Area or Area
b) If is the radius of a circle then:
- Perimeter of a semi circle
- Area of semi circle
c) If is the radius of a circle then:
- Perimeter of the quadrant
- Area of the quadrant
d) If and are radii of two concentric circles, then
- Area enclosed between the two circle
e) If is an equilateral triangle of side and height , then .
Also if and are the radii of the circumscribed and inscribed circles of , then and
- Circumference of the circumcircle of
- Area of the circumcircle
- Circumference of the inscribed circle
- Area of the inscribed circle
f) Important notes:
- If two circles touch internally, then the distance between their centers is equal to the difference of their radii.
- If tow circles touch externally. Then the distance between their centers is equal to the sum of their radii.
- The distance moved by a rotating wheel in one revolution is equal to the circumference of the wheel.
- The number of revolutions completed by a rotating wheel in one minute