tan(θ－π/8)
=tan[(θ＋π/8)－(π/4)]
=[tan(θ＋π/8)－tan(π/4)]/[1＋tan(θ＋π/8)tan(π/4)]
=[tan(θ＋π/8)－1]/[1＋tan(θ＋π/8)]
=[2－1]/[1＋2]
=1/3
则：tan(θ－π/8)=1/3