Cofunction and even/odd identities · proof ·what it is and when to use it

Proof: tan(−θ) = −tan θ

Tangent is odd — proved by quotient of an odd and an even function. Every line below says which rule it uses, so nothing has to be taken on faith.

The proof

Derived in two lines from the quotient identity.

1. tan(−θ) = sin(−θ)/cos(−θ)
   Quotient identity. · Quotient identity (tangent)
2. = −sin θ/cos θ
   Sine is odd, cosine is even. · Sine is odd, Cosine is even
3. = −tan θ
   Quotient identity again, backwards.

Where the proof stops applying

Both sides are undefined at the same angles — where a denominator in the proof reaches zero (for instance cos x = 0 in a tangent or secant form). Elsewhere the argument above holds for every real angle.

The same claim, checked numerically

A proof is not the same thing as a check, and this page shows both: the reasoning above, and the first few angles fed to the same engine behind the verifier. If they ever disagreed, the data would be broken — the build would fail before publishing (accuracy policy).

Related

• How would I find tan(−θ) = −tan θ myself? — the derivation, which is a different question from the proof.
• Tangent is odd: when to use it — the practical side.
• All cofunction and even/odd identities · proof index

sampled at 800 random anglesSource: OpenStax Precalculus, Ch. 7 · Paul's Online Math Notes, Trig Cheat Sheet · revised 2026-09-26 ·how we check ·accuracy policy ·report an error