已知0已知0
已知0[cos(β-π/4)]^2=[cosβcos(π/4)+sinβsin(π/4)]^2=[(cosβ)^2+(sinβ)^2]*[cos(π/4)]^2+2*cosβ*sinβ*cos(π/4)*sin(π/4)=1/2+1/2*sin2β=1/9故 sin2β=-7/90