令u=4x,则sin^3(4x)=sin^3(u);d[sin^3(4x)]/dx={d[sin^3(4x)]/du}{du/dx}={d[sin^3(u)]/du}{du/dx}=3sin^2(u)·cos(u)·{d(4x)/dx}=4·3sin^2(u)·cos(u)=12sin^2(4x)·cos4x)