Symbolic quadrant

"Strange changes and even changes" refers to the crime (? ∏/2+φ),? Parity. If it is an odd multiple of ∏/2, change sin (or cos) to cos (or sin). If it is an even multiple of ∏/2, there is no need to change it.

"Symbol looking at quadrant" refers to the image in the original trigonometric function. If it is positive, it is still positive. If it is negative, add a negative sign. Let me give you an example.

Such as sin(∏/2-φ), where φ is an acute angle. The coefficient before ∏/2 is 1, which is odd and needs to be changed to cos. This is the singularity.

∏/2-φ in the original trigonometric function is in the first quadrant. We know that the image of sinx is positive in the first quadrant and the second quadrant, so after it becomes cos, there is no need to change the sign, remove ∏/2 and leave φ.

To sum up, sin(∏/2-φ)=cosφ.

Another example is sin(∏-φ), where φ is an acute angle. If the coefficient before ∏/2 is 2, there is no need to change it to cos, but it is still.

Sin. This is an even constant.

In the original trigonometric function, ∏-φ is in the second quadrant, so it is positive. Remove ∏ and leave φ.

To sum up, sin(∏-φ)=sinφ.