Identities are "equations" that are always true.
For example, the equation sin(x) = cos(x) is true for x = pi/4 + kpi radians where k is any integer [ = 45 + 180k degrees], but for any other value of x the equation is not true.
By contrast, the equation sin2(x) + cos2(x) = 1 is true whatever the value of x. This is an identity.
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In trigonometry, identities are mathematical expressions that are true for all values of the variables involved. Some common trigonometric identities include the Pythagorean identities, the reciprocal identities, the quotient identities, and the double angle identities. These identities are used to simplify trigonometric expressions and solve trigonometric equations.
Unlike equations (or inequalities), identities are always true. It is, therefore, not possible to solve them to obtain values of the variable(s).
sin^2 (feta) + cos^2 (feta) = 1 sin (feta) / cos (feta) = tan (feta)
plane trigonometry spherical trigonometry
You start wit one side of the identity and, using logical steps, show that it is equivalent to the other side. Or, you start with both sides and show that they both equivalent to some common expression.