1. First of all, sina=cos(π/2-a) holds, but cos(π/2-a)=cos(a-π/2) is based on cosa=cos(-a), so sina=cos(a-π/2), so SIN (.

If you don't know or I made a mistake, let me know. The contact time is not long, and many of them may be unfamiliar. Also, do you personally think that is the range of finding f(3x)? Because it rarely comes out like this, although it has been encountered, it is generally a domain.