Trigonometric Functions

The Greek letters θ (theta) and φ (phi) are often used for angles. In the calculus it is convenient to measure angles in radians instead of degrees. An angle θ in radians is defined as the length of the arc of the angle on a circle of radius one (Figure 2.5.1). Since a circle of radius one has circumference 2π,

360 degrees = 2π radians.

Thus a right angle is

90 degrees = π/2 radians.