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
教你记公式的方法：
正余余正符号同,余余正正符号异
sin7π/18cos2π/9-sinπ/9sin2π/9的值
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