tanα=tan[(α-β)+β]=[tan(α-β)+tanβ]/[1-tan(α-β)tanβ]=1/3
tan2α=2tanα/(1-tan^2α)=3/4
tan(2α-β)=(tan2α-tanβ)/(1+tan2αtanβ)=1
∵α、β∈(0,π)
∴2α-β=3π/4或π/4或5π/4