MCR3U
SOLVING TRIG EQUATIONS
A trig equation contains a trig function and is only true for certain values of the variable.
Example 2 cos 𝜃 − 1 = 0
We are looking for the angle theta that satisfies the equation. Because the domain of cosine is all real
numbers, and the cosine graph is repetitive, there are actually an infinite number of angles that satisfy that
equation!
We will usually give you a restriction, for example we may ask for angles between 0° and 360°.
Primary Trigonometric Functions
y-sin
y-cos
y-tan
CAST Rule
When you are given a trig ratio, the CAST rule will help you determine where the terminal arm will be.
Examples
0
0
1. Solve cos  0.8667 for 0    360 .
MCR3U
SOLVING TRIG EQUATIONS
2. Solve sin 𝜃 = 0.2345 for 0° ≤ 𝜃 ≤ 360°.
3. Solve −2 cot 𝜃 − 1 = 2 for 0° ≤ 𝜃 ≤ 360°.