
# 3: Identities

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• 3.1: Basic Trigonometric Identities
Equations that are true for angles θ for which both sides of the equation are defined are called identities. In this section we will discuss several identities involving the trigonometric functions that are often used to simplify complicated expressions or equations.
• 3.2: Sum and Difference Formulas
We will now derive identities for the trigonometric functions of the sum and difference of two angles.
• 3.3: Double-Angle and Half-Angle Formulas
A special case of the addition formulas is when the two angles being added are equal, resulting in the double-angle formulas.
• 3.4: Other Identities
Though the identities in this section fall under the category of “other”, they are perhaps (along with $$\cos^2 θ+\sin^2 θ = 1$$) the most widely used identities in practice. It is very common to encounter terms such as $$\sin A +\ sin B \text{ or }\sin A \cos B$$ in calculations, so we will now derive identities for those expressions.
• 3.E: Identities (Exercises)
These are homework exercises to accompany Corral's "Elementary Trigonometry" Textmap. This is a text on elementary trigonometry, designed for students who have completed courses in high-school algebra and geometry. Though designed for college students, it could also be used in high schools. The traditional topics are covered, but a more geometrical approach is taken than usual. Also, some numerical methods (e.g. the secant method for solving trigonometric equations) are discussed.

Thumbnail: If $$\theta$$ is in QIII, then the legs of the right triangle formed by the reference angle have lengths $$|x|$$ and $$|y|$$ (we use absolute values because $$x$$ and $$y$$ are negative in QIII).