To change degrees to radians, the equivalent relationship 1° = π/180 radians is used, and the given number of degrees is multiplied by π/180 to convert to radian measure. Similarly, the equation 1 radian = 180°/π is used to change radians to degrees by multiplying the given radian measure by 180/π to obtain the degree measure.

The six basic trigonometric functions may be defined using a circle with equation *x* ^{2} + *y* ^{2} = *r* ^{2} and the angle θ in standard position with its vertex at the center of the circle and its initial side along the positive portion of the *x*‐axis (see Figure

**Figure 1 **Defining the trigonometric functions.

The trigonometric functions sine, cosine, tangent, cotangent, secant, and cosecant are defined as follows:

It is essential that you be familiar with the values of these functions at multiples of 30°, 45°, 60°, 90°, and 180° (or in radians, π/6, π/4, π/3, π/2, and π (See Table

The relationship between the angles and sides of a triangle may be expressed using the **Law of Sines** or the **Law of Cosines** (see Figure 2

**Figure 2 **Relations between sides and angles of a triangle.

**Table 1 Values of Sine, Cosine and Tangent at Common Angles**