In Figure 2.1.1 we see that a central angle of 90∘ cuts off an arc of length π2r, a central angle of 180∘ cuts off an arc of length πr, and a central an...In Figure 2.1.1 we see that a central angle of 90∘ cuts off an arc of length π2r, a central angle of 180∘ cuts off an arc of length πr, and a central angle of 360∘ cuts off an arc of length 2πr, which is the same as the circumference of the circle. Formally, a radian is defined as the central angle in a circle of radius r which intercepts an arc of length r, as in Figure 4.1.2.
