Covariance and variance both have the same denominator (n − 1), so this cancels out in the final formula.

Note, the formula shown right is the calculator way to calculate the slope of a simple bivariate regression. You could equally well use b = Σ(xy)/Σ(x²) where x=X- and y=Y-.

If you are obtaining the slope from an R-function such as lm, this slope is liable to be calculated rather differently because it is assumes you have one or more X-variables.