You measure angles using degrees and radians. You usually measure the lengths of sides using real numbers. You relate the measurements of angles to the measurements of sides using ratios. A set of six such ratios constitute the primary trigonometric ratios. You have already examined the ratio that generates the sine of the angle (theta) θ.
If you extend the discussion that began with the sine of the angle θ, you can then work forward to explore the ratios defined for the cosine and tangent of θ. You then move on from there to explore the ratios defined for cotangent, secant, and cosecant. Figure 12.8 illustrates the standard form of the right triangle with the sides explicitly identified.