f(x)=(sinX+sin3X)/(cosX+cos3X) ={2sin[(x+3x)/2]cos[(x-3x)/2]}/{2cos[(x+3x)/2]cos[(x-3x)/2]}=sin(2x)/cos(2x)=tan(2x)周期为π,是考虑cos(2x)cos(-x)≠0,2x≠kπ+π/2且x≠kπ+π/2参考：和差化积sin α+sinβ=...