TeX source:

S S Q_{\text {errar }}=\sum\left(X_{\mathrm{i} 1}-M_{1}\right)^{2}+\sum\left(X_{\mathrm{i} 2}-M_{2}\right)^{2}+\ldots+\sum\left(X_{\mathrm{ik}}-M_{\mathrm{k}}\right)^{2}