sin（π-x）=sinx,
cos（2π-x）=cosx,
tan（ -x+3/2π）=tan(π/2-x)=cotx=cosx/sinx,
cos（ - π- x）=cos(π-x)=-cosx,
则f(x)=cos²x/-cosx=-cosx,
f（- 31/3 π）=-cos(-31/3π)=-cos(-π/3)=-cosπ/3=-1/2.