TeX source:
\sigma_{M_{1}-M_{2}}=\sqrt{\frac{\sigma_{1}^{2}}{n_{1}}+\frac{\sigma_{2}^{2}}{n_{2}}}=\sqrt{\frac{\sigma^{2}}{n}+\frac{\sigma^{2}}{n}}=\sqrt{\frac{2 \sigma^{2}}{n}}