In the earlier chapters of my notes, the formula for $\hat{\beta_1}$ in simple linear regression was given as $$\frac{\hat{\sigma}}{\sqrt{\sum_{i=1}^{n}(x_i - \bar{x})^2}}$$.
However, in some later chapters, namely in discussion of the no-intercept model, the formula became $$\frac{\hat{\sigma}}{\sqrt{\sum_{i=1}^{n}x_i^2}}$$ instead.
Is it simply a typo, or is there a difference in formula where the no-intercept model is concerned?