sinπ/9=sin2π/18=cos(π/2-2π/18)=cos(9π/18-2π/18)=cos7π/18原式=sin7π/18cos2π/9-cos7π/18sin2π/9 （利用两角差的公式可知）=sin（7π/18-2π/9）=sin（7π/18-4π/18）=sin3π/18=sinπ/6=1/2教你记公...