This lecture covers the Fundamental Theorem of Integral Calculus, which states that if f is a continuous function on the interval [a, b], then the function G(x) = ∫f(t) dt with limits of integration from a to x is a primitive of f. The lecture revisits the concept of primitives, demonstrates the calculation of definite integrals using primitives, and explores the estimation of integrals. It also introduces the technique of integration by change of variables, providing a detailed explanation and examples.