Mathematics
Taylor Series Expansion and Its Error Bounds
Quick fact
The Taylor series of a function about a point is unique—if a function has a power series representation at that point, it must be the Taylor series, making it a foolproof way to construct polynomial approximations.
Why this is interesting
Projectile motions, planetary orbits, and smartphone algorithms all rely on approximating complicated functions with simple polynomials—how can a few derivative values unlock this magic?