Trigonometric function in high school mathematics.

Odd even, the sign of quadrant is the formula to change the sign of cos and sin. The details are as follows: sin(3π/2+α)=-cosα, and sin(π/2+α)=cosα. "Odd becomes even" means that when the number added after α is an odd multiple of π/2, the name will change (that is, if it turns out to be sin). "Symbolically looking at the quadrant" means finding sin(3π/2+α), and taking α as the acute angle of the first quadrant, then 3π/2+α is the angle of the fourth quadrant, and the sin of the fourth quadrant is negative, so it can be simplified as -cosα. (Note: Simplification symbols are based on values that need to be simplified. Just like the example I just gave, I only look at the symbol of sin(3π/2+α) and don't look at the symbol of cos(3π/2+α). Remember! ! ! ! )