sina=5/13，π/2因为，sin²a+cos²a=1，且cosa<0所以，cosa＝-12/13 sin(a-π/3)=(sina)(cosπ/3)-(cosa)(sinπ/3)=(5/13)(1/2)-(-12/13)(√3/2)=(5+12√3)/26 π/2sin(a/2)>0又，cosa=1-2sin²(a/2)sin²(a/2)=(1-cosa)/2=(1+12/13)/2=25/26所以，sin(a/2)=(5√26)/26
