mathlib

ac3c4787 - feat(archive/100-theorems-list/9_area_of_a_circle): area of a disc (#6374)

feat(archive/100-theorems-list/9_area_of_a_circle): area of a disc (#6374) Freek № 9: The area of a disc with radius _r_ is _πr²_. Also included are an `of_le` version of [FTC-2 for the open set](https://leanprover-community.github.io/mathlib_docs/find/interval_integral.integral_eq_sub_of_has_deriv_at') and the definition `nnreal.pi`. Co-authored by @asouther4 and @jamesa9283. Co-authored-by: Andrew Souther <asouther@fordham.edu> Co-authored-by: Benjamin Davidson <68528197+benjamindavidson@users.noreply.github.com> Co-authored-by: Heather Macbeth <25316162+hrmacbeth@users.noreply.github.com>