We discussed how to solve Ax=b, and it only works when A is square (same number of equations as variables).
Consider the case of finding the best line to fit a set of points. We want to find x1 and x2 such that x1 ni + x2 = ti where ni is problem size and ti is time.
We can't solve this exactly (in general) if we have more than two data points (which is the only useful case anyway!).
