tana=-1／2
故tan2a=2tana/[1-(tana)^2]=2×(-1/2)/[1-(-1/2)^2]=-4/3
则
(sin2a+2cos2a)／(4cos2a-4sin2a) 分子分母同除以cos2a
=(tan2a+2)／(4-4tan2a)
=(-4/3+2)／[4-4×(-4/3)]
=1/14