(sin^2x+2sinxcosx+cos^2x)/(cos^2x-sin^2x)
上下同除cos^2x
=(tan^2x+2tanx+1)/(1-tan^2x)
=(3^2+2*3+1)/(1-3^2)
=(9+6+1)/(1-9)
=16/(-8)
=-2