Trigonometry 4.2: Double angle formula

You’ll learn

• Use the double-angle formulas: sin 2A = 2 sin A cos A and tan 2A = 2 tan A/(1 − tan²A)
• Pick the right form of cos 2A (cos²A − sin²A, 2cos²A − 1, 1 − 2sin²A) for your variable
• Express sin 3A and cos 3A in a single variable by splitting 3A = 2A + A
• Use power reduction sin²A = (1 − cos 2A)/2 to find values like sin 15° from cos 30°