答：
tanx=-3/4
2+sinxcosx-(cosx)^2
=2(sinx)^2+2(cosx)^2+sinxcosx-(cosx)^2
=[ 2(sinx)^2+(cosx)^2+sinxcosx ] / [ (sinx)^2+(cosx)^2 ] 分子分母同时除以(cosx)^2
=[ 2(tanx)^2+1+tanx ] / [ (tanx)^2+1]
=[ 2*(-3/4)^2+1-3/4 ] / [ (-3/4)^2+1 ]
=(9/8+1/4) / (9/16+1)
=(11/8) / (25/16)
=22/25