TeX source:

\begin{split} \Pr\{\text{even}\} &= \langle00|\rho_{\text{EPR}}|00\rangle + \langle11|\rho_{\text{EPR}}|11\rangle \\ &= \dfrac{1}{2}\langle00|(|00\rangle\langle00| + |11\rangle\langle11| + |00\rangle\langle11| + |11\rangle\langle00|)|00\rangle + \langle11|\rho_{\text{EPR}}|11\rangle \\ &= \dfrac{1}{2}(\langle00| + \langle11|)|00\rangle + \langle11|\rho_{\text{EPR}}|11\rangle = \dfrac{1}{2} + \dfrac{1}{2} = 1, \end{split}