About

Lists of trig identities are easy to find. Deciding which one to use in front of a specific expression is the part nobody teaches — that gap is what this site is for.

What is here

• A chooser: type an expression, see which rule matches, why it matches and what your expression becomes after applying it.
• A verifier that shows its working: how many angles were sampled and a concrete counterexample when the claim is false.
• 44 identities, each with a page for what it is for, a proof, a derivation and worked examples.
• A printable sheet and practice problems with answers.

How it is built

One small expression engine (parser, sampler, rewriter) written once and used in three places: it runs the tools in your browser, it generates the numbers printed on every page, and it is the QA gate that decides whether a data edit can be published at all. A static site with no account, no database and no tracking — the URL is the whole state.

What it is not

• Not a symbolic solver. It will not do your algebra for you; it tells you which identity applies and checks claims numerically.
• Not a proof assistant. The proofs are ordinary mathematics written out, not machine-verified logic.
• Not a course. For syllabus and exam decisions, your teacher's approved list wins.

Part of a two-site family: outerdiameter.com (pipe outer-diameter reference) uses the same engine-shaped approach on a different subject.