f'(x)=lim(Δx-->0)Δy/Δx
=lim(Δx-->0)[sin(3x+3Δx+1)-sin(3x+1)]/Δx
=lim(Δx-->0) [2cos(3x+3/2*Δx+1)sin(3Δx/2)]/Δx
=cos(3x+1)*lim(Δx-->0)3sin(3Δx/2)/(3Δx/2)
根据重要极限lin(x-->0)sinx/x=1
∴lim(Δx-->0)sin(3Δx/2)/(3Δx/2)=1
∴f'(x)=cos(3x+1)*lim(Δx-->0)3sin(3Δx/2)/(3Δx/2)
=3cos(3x+1）