tan2(a-β)=2tan(a-β)/(1-tan^2(a-β))=1/(1-1/4)=4/3
tan(2a-β)=tan[(2a-2β)+β]
=[tan2(a-β)+tanβ]/[1-tan2(a-β)*tanβ]
=(4/3-1/7)/(1+4/21)
=25/25
=1
a∈(0,π/4),β∈(0,π),tanβ=-1/7
tan2(a-β)=2tan(a-β)/(1-tan^2(a-β))=1/(1-1/4)=4/3
tan(2a-β)=tan[(2a-2β)+β]
=[tan2(a-β)+tanβ]/[1-tan2(a-β)*tanβ]
=(4/3-1/7)/(1+4/21)
=25/25
=1
a∈(0,π/4),β∈(0,π),tanβ=-1/7