y= cos(x+2π/9)sin（x+π/18）
=1/2[（sin(x+x+2π/9+π/18)-sin(2π/9-π/18)]
=1/2[sin(2x+（5π /18)）-sinπ/6]
=1/2[sin(2x+（5π /18))-1/2）
=1/2sin(2x+（5π /18))-1/4
当sin(2x+（5π /18))=-1时,即2x+5π/18=（-π/2）+2kπ时,有最小值,
y最小值=-1/2-1/4=-3/4