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
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