Proving the Double and Half Angle Formulas for Trigonometry (Precalculus - Trigonometry 27)
Professor Leonard · 27:16
This lesson proves all the double-angle and half-angle trig formulas using only two starting points: the sum formulas and the Pythagorean identity, plus taking square roots at the end. You get sin 2θ = 2 sin θ cos θ,...