trigonometry
The Pythagorean identity is just Pythagoras, divided
sin²α + cos²α = 1 is not a new fact to memorise. It is the Pythagorean theorem with both sides divided by c² — and two more useful identities fall straight out of it.
Lessons
One idea per lesson, two to five minutes each. Every formula is derived, never just stated.
trigonometry
trigonometry
The three special angles, from two triangles
Where the exact values of sin, cos and tan at 30°, 45° and 60° come from — derived from half an equilateral triangle and half a square, not memorised.
trigonometry
Radians: why 180° = π, and what a radian actually is
A radian is not a conversion factor to memorise — it is the angle standing on an arc as long as the radius. Everything else follows from one line.
Free download
The trigonometry formula sheet
Every trigonometry formula on nine printable pages: exact values, the reduction rules, every identity, and a full-page unit circle chart. No sign-up, just the PDF.
Download the PDF