tan(2β)=1/3*2/(1-1/3*1/3)=3/4
tan(a+2β)=(1/7+3/4)/(1-1/7*3/4)
=(4+21)/(28-3)
=1
tan(2β)>0且0<2β<π
所以0<2β<π/2
tan(a+2β)>0且0所以0所以a+2β=π/4