Assume M = N1 + N2 is a module, where N1, N2 are noetherian modules.
0 "\\to" N1 "\\xrightarrow[]{i}" N1 "\\oplus" N2 "\\xrightarrow[]{j}" N2 "\\xrightarrow[]{}" 0
where i(n)=(n,0) and j(n,m)=m. As it is an exact sequence, the module in the middle, i.e. N1 "\\oplus" N2, is noetherian if the two other modules are noetherian.
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