How do you understand the oral judgment "Odd changes to even, the same, the symbols look at the quadrants"? Can you tell me more about it?

First, consider α as an acute angle. The so-called odd and even numbers are the coefficients of π/2.

If it is an odd number, the name needs to be changed, that is, sin becomes cos. For example, sin (π/2-α) = cosα. The coefficient of π/2 here is 1, an odd number, so the right side of the equal sign must be Changed its name to cosα. Then decide whether it is cosα or -cosα, that is, look at the symbol quadrant. When you think of α as an acute angle, -α is in the fourth quadrant, and the angle [π/2-α] should be in the first quadrant. The sin value of the first quadrant angle should be a positive number, which is the sin on the left side of the equal sign ( The value of π/2-α) is positive, so the number on the right must also be positive. α is regarded as an acute angle, and cosα is positive, so sin(π/2-α)=cosα

This is the truth. If I am not clear, you can ask me again and I will add more