du refers to an infinitesimally small change in the variable u. It typically arises when performing substitutions during integration.
Δu: Represents the finite change in u, obtained by subtracting the initial value of u from its final value.
∫du: Indicates integration with respect to u, implying that we are finding the antiderivative of a function with respect to u.
du/dt: Denotes the derivative of u with respect to time t, showing how quickly or slowly u changes over time.