tanx=sinx/cosx=3则 cosx,sinx均不为零,
所以（4sinx-2cosx）/(5cosx+3sinx)
=(4sinx/cosx-2)/(5+3sinx/cosx)
=(4tanx-2)/(5+3tanx)
=(4*3-2)/(5+3*3)
=10/14
=5/7