L1的倾斜角α L2的倾斜角β
α=2β
tgα=4tgβ
tg2β=4tgβ
2tgβ/(1-(tgβ)^2)=4tgβ
tgβ不等于0
所以1-(tgβ)^2=1/2
(tgβ)^2=1/2
mn=tgα*tgβ=4(tgβ)^2=2