tan(2β)=2tanβ/[1-(tanβ)^2]=2*(1/3)/[1-(1/3)^2]=3/4
tan(α+2β)=[tanα+tan(2β)]/[1-tanαtan(2β)]=[(1/7)+(3/4)]/[1-(1/7)*(3/4)]=1
所以：α+2β=45度