Reciprocal and quotient identities · proof ·what it is and when to use it

Proof: cot θ = 1/tan θ

Reciprocal identity (cotangent) — proved by definition. Every line below says which rule it uses, so nothing has to be taken on faith.

The proof

The third reciprocal pair, and the only one where the ratio form matters twice.

1. tan θ = opp/adj
   TOA. · Quotient identity (tangent)
2. adj/opp = 1/tan θ
   Reciprocal of both sides.
3. cot θ = adj/opp
   Definition of cotangent — and also cos θ/sin θ. · Quotient identity (cotangent)

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 cot θ = 1/tan θ myself? — the derivation, which is a different question from the proof.
• Reciprocal identity (cotangent): when to use it — the practical side.
• All reciprocal and quotient 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