Determine which side is the unknown. Trigonometry only works on triangles with right angles (90 degree angles). Draw the triangle on a piece of paper so that the 90 degree angle is on the bottom left hand side. Label the angles and lengths that are known.
Determine which trigonometric formula to use. There are three basic formulas that can be used. They are:
Tangent(theta) = opposite / adjacent
Sine(theta) = opposite / hypotenuse
Cosine(theta) = adjacent /hypotenuse
If the triangle's right angle is at the bottom left (as directed in step 1), the opposite side is the vertical line on the left, the adjacent side is the horizontal line and the hypotenuse is the diagonal line. Theta is defined as being the angle between the hypotenuse and the adjacent lines.
It is only possible to find one unknown, so the relevant equation above should be chosen. For example, to find the adjacent, while theta and opposite are known, the cosine (abbreviated "cos") formula should be used.
Type the relevant formula into a scientific calculator. Following this example, if the adjacent is unknown, theta is 20 degrees, and opposite is 3 cm, the following procedure would be used.
1) Rearrange the formula to make the adjacent the subject of the equation
e.g. cos(theta) = adjacent / hypotenuse
multiply both sides by hypotenuse, which gives
Adjacent = hypotenuse x cos(theta)
2) Type the equation into the calculator. For this example the following would be typed into the calculator: 3 x cos(20). This should give an answer of 2.81 cm for the adjacent side.