Double and Half-Angle Identities

cos(a-b)

cosacosb + sinasinb

cos(a+b)

cosacosb - sinasinb

sin(a+b)

sinacosb + cosasinb

sin(a-b)

sinacosb - cosasinb

tan(a+b)

(tana+tanb)/(1-tanatanb)

tan(a-b)

(tana-tanb)/(1+tanatanb)

sin2θ

2sinθcosθ

cos2θ

cos²θ-sin²θ

1-2sin²θ

2cos²θ-1

tan2θ

2tanθ/1-tan²θ

sin(θ/2)

±√\[(1-cosθ)/2\]

cos(θ/2)

±√\[(1+cosθ)/2\]

tan(θ/2)

(1-cosθ)/sinθ