dy/dx=(3x^3-4x)'(2x+1)+(3x^3-4x)(2x+1)'
=(9x^2-4)(2x+1)+(3x^3-4x)(2)
=(18x^3-8x+9x^2-4)+(6X^3-8x)
=24x^3+9x^2-16x-4
或者y=(3x^3-4x)(2x+1)=6X^4-8X^2+3x^3-4x
dy/dx=24x^3-16x+9x^2-4