Numbering rule: regard α as an acute angle (note that it is "regarded"), and take the sign of trigonometric function according to the obtained angle. That is, "Like a finite number, the symbol looks at the quadrant" (or "Even if it changes singularly, the symbol looks at the quadrant").
In Kπ/2, if k is even, the function name remains the same, and if it is odd, the function name becomes the opposite function name. See the symbol of the quadrant where α is in the original function. There is a formula about symbols; One is all positive, the other is sine, the third is tangent and the fourth is cosine, that is, the first quadrant is all positive, the second quadrant is all sine, the third quadrant is all cotangent and the fourth quadrant is all cosine. Or ASTC for short, that is, all, sin, tan+cot and cos are positive in turn.
For example: 90+α. Naming: 90 is an odd multiple of 90, and the complementary function should be taken; Note: If α is regarded as an acute angle, then 90+α is the second quadrant, and the sine of the second quadrant is positive and the cosine is negative. So sin (90+α) = cos α, cos (90+α) =-sin α?
Another formula is "vertical variation and horizontal variation, and the sign depends on the quadrant", for example: SIN (90+α), the terminal edge of 90 is on the vertical axis, so the function name becomes the opposite function name, that is, cos, so SIN (90+α) = COS α.