What are the double-angle and half-angle identities?

Answer 1

sin 2θ = 2(sin θ)(cos θ)

cos 2θ = (cos θ)2 - (sin θ)2

cos 2θ = 2(cos θ)2 - 1

cos 2θ = 1 - 2(sin θ)2

tan 2θ = 2(tan θ)/[1 - (tan θ)2]

sin θ/2 = ±√[(1 - (cos θ))/2]

cos θ/2 = ±√[(1 + (cos θ))/2]

tan θ/2 = ±√[(1 - (cos θ))/(1 + (cos θ))] ; cos θ ≠ -1

tan θ/2 = [1 - (cos θ)]/(sin θ)

tan θ/2 = (sin θ)/[1 + (cos θ)]