TeX source:

\begin{split} &\text{tr}\left\{(\mathbf{M}_{+A}\otimes\mathbf{I}_B)\left( \dfrac{1}{2}|0\rangle\langle0|_A\otimes|0\rangle\langle0|_B + \dfrac{1}{2}|1\rangle\langle1|_A\otimes|1\rangle\langle1|_B \right)\right\} \\ &= \text{tr}\left\{\dfrac{1}{2}|+\rangle\langle+|0\rangle\langle0|\otimes|0\rangle\langle0| + \dfrac{1}{2}|+\rangle\langle+|1\rangle\langle1|\otimes|1\rangle\langle1|\right\} = \dfrac{1}{2}. \end{split}