Mathematics
Linear Regression via the Normal Equations
Quick fact
The normal equations give the least-squares solution directly in one step, unlike iterative methods such as gradient descent. For a simple straight line, they reduce to basic formulas for the slope and intercept using only the means and variances of the data.
Why this is interesting
You can draw a line through almost any scatter plot, but how do you find the best one? And why does the answer come out of a single, simple formula?