sin(-17π/3)
=-sin(17π/3)
=-sin(4π+π+2π/3)
=-sin(π+2π/3)
=-[-sin(2π/3)]
=sin(2π/3)=√3/2;
cos（-17π/3)
=cos(17π/3)
=cos(5π+2π/3)
=cos(4π+π+2π/3)
=cos(π+2π/3)
=-cos(2π/3)
=-1/2.