sin(α-β)cosα-coa(α-β)sinα=2/3
sin[(α-β)-α]=2/3
sin(α-β-α)=2/3
sin(-β)=2/3
-sinβ=2/3
sinβ=-2/3
cos2β
=1-2(sinβ)^2
=1-2*4/9
=1/9
cos4β
=2(cos2β)^2-1
=2*1/81-1
=-79/81
tan(2x+y)=3,tanx=-5 求tan(x+y)
tan(x+y)
=tan[(2x+y)-x]
=[tan(2x+y)-tanx]/[1+tan(2x+y)tanx]
=(3-5)/(1-3*5)
=-2/(-13)
=2/13