Solve linear regression with the normal equations: derive the closed form and implement it stably.

A from-scratch test of the least-squares closed form and the numerical traps in it. The signal is deriving the normal equations, knowing why you never explicitly invert, and reaching for lstsq or the SVD instead. Here is the implementation.

Updated Aug 2026 · Grounded in real Applied AI Engineer interview loops and written to a senior-engineer editorial bar.