f(x)=sin(π-x)cos(2π-x)/sin(π/2+x)tan(π+x)
=sinxcosx/cosxtanx
=cosx
f（31π/3）=cos(10π+π/3)=cosπ/3=1/2它是tan（π+a）不是x(x)=sin(π-x)cos(2π-x)/sin(π/2+x)tan(π+a)=sinxcosx/cosxtana =sinx/tana f(31π/3)=sin(31π/3)/tana=√3/(2tana)