Freshman-freshman four-substitution integral problem. Thank you, thank you!
For the problem of 12, the power drop formula of cosine 2 times angle Cos2x = 2 (Cosx) 2- 1 is used again.
Question 13 uses the "product and difference formula" of trigonometric functions. Sinacosb = [sin (a+b) x+sin (a-b) x]/2. Middle school students are not required to remember this formula, but universities require it. This is a disjointed problem.