是y=(2x+1)/(4x-9)吗?
dy/dx=[(2x+1)'(4x-9)-(4x-9)'(2x+1)]/(4x-9)^2
=[2(4x-9)-4(2x+1)]/(4x-9)^2
=(8x-18-8x-4)/(4x-9)^2
=-22/(4x-9)^2.