For example, if a circle has twice the diameter of another circle it will also have twice the circumference, preserving the ratio C/d. This definition of π implicitly makes use of flat (Euclidean) geometry; although the notion of a circle can be extended to any curved (nonEuclidean) geometry, these new circles will no longer satisfy the formula π = C/d"  как можно в двух строках дать определение постоянной, но не дать определение неевклидовой геометрии?