tgβ+1/tgβ=5/2 2(tgβ)^2-5tgβ+2=0 (2tgβ-1)(tgβ-2)=0 tgβ=2或tgβ=1/2
(sinβ)^2/[1-(sinβ)^2]=4或1/4 sinβ=-2/根号5或-1/根号5