The document discusses Pythagorean identities. It introduces the Pythagorean theorem that relates the sides of a right triangle, and defines trigonometric functions in terms of angles. It then lists four fundamental Pythagorean identities: 1) sin2θ + cos2θ = 1, 2) 1 + tan2θ = sec2θ, 3) cot2θ + 1 = csc2θ, and 4) sec2A - 1 = csc2A tan2A. It provides examples and homework problems related to these identities.