Show that if $M$ is a direct sum of $M_1$ and $M_2$ then $M/M_1$ is isomorphic to $M_2$ and $M/M_2$ is isomorphic to $M_1$.
$M=M_1\oplus M_2$. Then show that $M/M_1$ is isomorphic to $M_2$ and $M/M_2$ is isomorphic to $M_1$
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