20 10 formulas of trigonometric functions's strange problem of mathematical proof in Sichuan college entrance examination
For that problem, I remember proving the sum angle formula of trigonometric function for a lifetime. Okay, what about trigonometric functions? I was confused at that time, so I gave up the original question as follows: Proof: COS(a+b)= COSA COSB- Sina SINB SINB (a+b) = Sina COSB-COSA SINB.