Trigonometry

Learn Trigonometry

From right triangles to the unit circle to identities. This is the bridge that carries you into calculus.

Start Trigonometry free → Browse all subjects

Free to start · adaptive placement finds your level · reviews timed to your own forgetting.

35 lessons in Trigonometry

Right-triangle trig (SOHCAHTOA)The unit circleGraphs of sin and cosTrig identitiesInverse trig & their domainsSolving trig equationsLaw of cosinesLaw of sinesArea of triangle via trigSum & difference formulasDouble-angle formulasHalf-angle formulasGraphs of tan, sec, csc, cotPower-reduction & multiple-angle expansionProduct-to-sum & sum-to-productProving trig identitiesHarmonic addition: a·sin + b·cosSimple harmonic motion & sinusoidal modelingPolar curvesTrig form of complex numbersDe Moivre's theoremRoots of unityEuler's formula & trig identitiesLimit $\sin x / x \to 1$ (geometric proof)Solving triangles: law of tangents & half-angleHyperbolic functionsTaylor series of sine & cosineGregory's series & the Leibniz formula for πThe tangent half-angle substitutionChebyshev polynomialsFourier series (introduction)Spherical trigonometry previewSpherical trigonometryThe navigator's trig functionsThe Gudermannian: circular meets hyperbolic without i

Built to be understood — and remembered.

Every idea is taught with motivation and a worked example before the drills, and an FSRS spaced-repetition engine schedules each review for the day its model predicts you would forget it. A short placement check finds what you already know, so you start Trigonometry exactly where it's useful.

Start Trigonometry →