tanα=-1/3
tan2α=2tanα/(1-tan^2 α)=(-2/3) / (1-1/9)=-3/4
tan(2α+β)=(tan2α+tanβ)/(1-tan2α*tanβ)
=(-3/4-1/7)/(1-3/4*1/7)
=(-25/28) / (25/28)
=-1
π/2<α<π,所以,π<2α<2π
tan2α<0,所以,3π/2<2α<2π
-π<β<0,tanβ=-1/7<0,所以,-π/2<β<0
所以,π<2α+β<2π
而：tan(2α+β)=-1
所以, 2α+β=7π/4