Arguably one of the more challenging concepts to learn in integral calculus, integration by parts is the process by which you find the integral of 2 expressions multiplied together. The most common formula for doing this is "udv = uv -integral(vdu)," but an easier way to view it is "uv = (u * integral(v)) - (integral(u' - integral(v))"
Integration by parts is a very tricky concept to learn at first, but with some time and practice can become easier to do and often amusing for calculus students.