Double angle formula: tan 2θ = 2 tan θ / (1 − tan²θ)
tan 2θ = 2 tan θ / (1 − tan²θ), from the tangent sum formula with β = α. It is undefined when tan²θ = 1, i.e. at odd multiples of π/4, which is exactly where 2θ lands on an asymptote of tangent.
Related Double Angle identities
Common questions
What is the tan 2θ identity?
tan 2θ = 2 tan θ / (1 − tan²θ), from the tangent sum formula with β = α. It is undefined when tan²θ = 1, i.e. at odd multiples of π/4, which is exactly where 2θ lands on an asymptote of tangent.
Which identities are related to tan 2θ?
tan 2θ belongs to the Double Angle group. The identities used alongside it most often are sin 2θ and cos 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.