Cavalieri derived his formula for the area underneath a parabola by utilizing a specific ratio related to the sum of squares. The key formula involved is the sum of the first m squares, expressed as $$\sum_{k=1}^m k^2 = \frac{m(m + 1)(2m + 1)}6$$. By substituting this sum into his initial equation, he was able to arrive at the desired result. This mathematical approach highlights the connection between geometric areas and algebraic summation. Understanding this derivation is crucial for grasping Cavalieri's contributions to calculus and geometry.