The method of exhaustion is a method of finding the area of a shape by inscribing inside it a sequence of polygons whose areas converge to the area of the containing shape. If the sequence is correctly constructed, the difference in area between the n-th polygon and the containing shape will become arbitrarily small as n becomes large. As this difference becomes arbitrarily small, the possible values for the area of the shape are systematically "exhausted" by the lower bound areas successively established by the sequence members. The idea originated with Antiphon, although it is not entirely clear how well he understood it. The theory was made rigorous by Eudoxus. The first use of the term was in 1647 by Grégoire de Saint-Vincent in Opus geometricum guadraturae circuli et sectionum coni.
The method of exhaustion typically required a form of proof by contradiction, known as reductio ad absurdum. This amounts to finding an area of a region by first comparing it to the area of a second region (which can be “exhausted” so that its area becomes arbitrarily close to the true area). The proof involves assuming that the true area is greater than the second area, and then proving that assertion false, and then assuming that it is less than the second area, and proving that assertion false, too.
The method of exhaustion is seen as a precursor to the methods of calculus. The development of analytical geometry and rigorous integral calculus in the 17th-19th centuries (in particular a rigorous definition of limit) subsumed the method of exhaustion so that it is no longer explicitly used to solve problems.
Archimedes used the method of exhaustion as a way to compute the area inside a circle by filling the circle with a polygon of a greater and greater number of sides. The quotient formed by the area of this polygon divided by the square of the circle radius can be made arbitrarily close to π as the number of polygon sides becomes large, proving that the area inside the circle of radius r is πr2, π being defined as the ratio of the circumference to the diameter. Incidentally, he also provided the celebrated bounds 3+10/71 < π < 3 + 1/7 comparing the perimeters of the circle with the perimeters of the inscribed and circumscribed 96-sided regular polygons.
Other results he obtained with the method of exhaustion included
A new form of the method of exhaustion provides a formula to evaluate the definite integral of any continuous function:
This formula can be useful when no elementary antiderivative exists. It can also be useful for teaching integral calculus.