Double angle formula: sin 2θ = 2 sin θ cos θ
sin 2θ = 2 sin θ cos θ, which drops straight out of the sine sum formula with β = α. It is the substitution that collapses a product of a sine and a cosine into a single term, and the one that makes ∫ sin θ cos θ dθ tractable.
Related Double Angle identities
Common questions
What is the sin 2θ identity?
sin 2θ = 2 sin θ cos θ, which drops straight out of the sine sum formula with β = α. It is the substitution that collapses a product of a sine and a cosine into a single term, and the one that makes ∫ sin θ cos θ dθ tractable.
Which identities are related to sin 2θ?
sin 2θ belongs to the Double Angle group. The identities used alongside it most often are cos 2θ and tan 2θ.
Practise it
Reading a formula is not the same as recalling it under time pressure. The workspace drills Double Angle questions in the quiz, and the triangle solver shows a worked solution for any triangle you type in.
Or browse all 45 identities in the full reference, or read the trigonometry guides for the method behind them.