Sometimes, I don't know how to present an answer well. For [itex] sin (x+y)=sin (5x+z) [/itex] For y and z are constant. You may subsitute some value into it. If the range of x is restricted to [itex] -\pi < x < \pi [/itex], naturally we will use [2pi-(5x+z)] or [pi-(5x+z)]. However, in some situation as I mentioned before, the required x value cannot be found in this way. Perhaps, we need to use [6pi-(5x+z)]=x+y instead to find out the value in the required range. How should I present? Generally, no one subs. pi and 2pi , then 5 pi , it's very strange, some marks maybe conserved for this reason.