Trigonometric Identities - Purplemath
In mathematics, an "identity" is an equation which is always true. These can be ... sin2(t) + cos2(t) = 1 tan2(t) + 1 = sec2(t) 1 + cot2(t) = csc2(t) ... cos(2x) = cos2(x) – sin2(x) = 1 – 2sin2(x) = 2cos2(x) – 1....