en.wikipedia.org/wiki/List_of_trigonometric_identities

In mathematics, **trigonometric identities** are equalities that involve trigonometric
functions and are true for every value of the occurring variables where both sides
...

www.sosmath.com/trig/Trig5/trig5/trig5.html

Reciprocal **identities**. displaymath161. Pythagorean **Identities**. displaymath162.
Quotient **Identities**. displaymath163. Co-Function **Identities**. displaymath164.

www.purplemath.com/modules/idents.htm

Lists the basic **trigonometric identities**, and specifies the set of **trig identities** to
keep track of, as being the most useful ones for calculus.

www.mathwords.com/t/trig_identities.htm

**Trig Identities**. Identities involving trig functions are listed below. Pythagorean
Identities. sin^{2} θ + cos^{2} θ = 1. tan^{2} θ + 1 = sec^{2} θ. cot^{2} θ + 1 = csc^{2} θ ...

www.clarku.edu/~djoyce/trig/identities.html

You have seen quite a few **trigonometric identities** in the past few pages. It is
convenient to have a summary of them for reference. These identities mostly refer
to ...

www.khanacademy.org/math/precalculus/trig-equations-and-identities-precalc/using-trig-identities-precalc/v/examples-using-pythagorean-identities-to-simplify-trigonometric-expressions

Rewrite trigonometric expressions using the Pythagorean identity and other
**trigonometric identities**.

www.khanacademy.org/math/precalculus/trig-equations-and-identities-precalc/using-trig-identities-precalc/a/trig-identity-reference

Look up AND understand all your favorite **trig identities**.

math2.org/math/trig/identities.htm

sin(theta) = a / c. csc(theta) = 1 / sin(theta) = c / a. cos(theta) = b / c. sec(theta) = 1 / cos(theta) = c / b. tan(theta) = sin(theta) / cos(theta) = a / b. cot(theta) = 1/ ...

www.themathpage.com/atrig/trigonometric-identities.htm

Pythagorean **identities**. Sum and difference formulas. Double angle formulas.
Half angle formulas. Products as sums. Sums as products.