The antiderivative of ln x is x ln x - x + C, where C is the constant of integration. This can be found using the method of integration by parts, where u = ln x and dv = dx. The derivative of u is 1/x, and the integral of dv is simply x. Using the formula for integration by parts, we get:

∫ln x dx = x ln x - ∫(1/x) x dx

Simplifying the right-hand side gives:

∫ln x dx = x ln x - x + C

where C is the constant of integration.