Radian Measure

In some calculations in circle geometry, radians are preferred to degrees.

An angle of 1 radian is the angle subtended by an arc with the same length as the radius shown below.

Conversions between radians and degrees

Arc Length

The formula to find the arc length is illustrated below:

Here, r is the radius and θ is the angle in radians.

If you had to find the arc length of a sector with a radius of 5cm and an angle of 80°, you would answer the question using this method - shown below.

Area of a Sector

The area of a sector can be found using this formula.

If we had to find the area of the sector in the previous example, we would use a method like this.