The area of a circle, we know, is
A = πr2
where r is the radius of the circle. Most of us first learned this formula in school with the justification that teacher said so, take it or leave it, but you’d better take it and learn it by heart; the formula is, in fact, an example of the brutality with which mathematics is often taught to the innocent.
[A History of Pi, Petr Beckman, p. 17]