-17π/6=-18π/6 + π/6
=-3π +π/6
因此：
tan(-17π/6)=tan(-3π +π/6)=tan(-π+π/6)=tan(-5π/6)=-tan(5π/6)=-tan(π - π/6)=tan(π/6)
=√3/3
-31π/4=-32π/4+π/4=-8π+π/4
因此：
tan(-8π+π/4)=tanπ/4=1