Because the coefficient before 1/4π-x x is negative, when the monotonicity of F(a)=sina is directly used, the coefficient before x should be turned into a positive number, as follows: f (x) = sin (1/4π-x) =-sin (x-1.

F(x)=sin(x- 1/4π) monotone subtraction interval, as long as X-1/4π is placed in the interval of (1/2π+2kπ 3/2π+2kπ), it is calculated that X belongs to (3/4π+2kπ).

When dealing with this kind of problem, we should pay attention to the positive and negative coefficients in front of X. If it is negative, we should first use the trigonometric formula to turn the negative sign into a positive sign, and then set it in the interval to solve it. If the negative sign is not handled in advance, the above situation will occur. I hope it will help LZ. In addition, this problem does not necessarily need guidance. Derivation is a complex non-basic function that does not know the image and basic properties, so it is more suitable for derivation.