The most basic thing is to master the operation formula between basic functions. Needless to say, you know this, and then learn to draw correctly, judge the range of interval and integral, and introduce custom function characteristic function when necessary. In short, the problem of function is lively, so we must learn to classify and transform.

Odd couples, symbols look at quadrants. Example: sin(π-x) can be converted into sinx if π/2 is an even multiple, and then it can be converted into cos symbol. If π-x regards x as an acute angle, π-x is positive in the second quadrant, so it can be converted into positive sinx, and cos is positive in the first and fourth quadrants. The conversion rules are the same as above.