cos(x+π/12)=-5/13,x是第三象限角则：x+π/12还是第三象限角所以,易得sin(x+π/12)=-12/13x-π/6=x+π/12-π/4所以,cos(x-π/6)=cos[(x+π/12)-π/4]=cos(x+π/12)cos(π/4)+sin(x+π/12)sin(π/4)=(-5/13)(√2/2)+...