If G isn't <a> then b is not in <a>, and if G isn't <b> then a isn't in <b>. In other words <a>∩<b> = 1. Since no proper subgroup of G contains {a,b}, the group generated by a and b, and since G is abelian then G = {a,b} ≈<a>x<b>≈Z_m x Z_n, where m = ord(a) and n = ord(b).
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If G isn't <a> then b is not in <a>, and if G isn't <b> then a isn't in <b>. In other words <a>∩<b> = 1. Since no proper subgroup of G contains {a,b}, the group generated by a and b, and since G is abelian then G = {a,b} ≈<a>x<b>≈Z_m x Z_n, where m = ord(a) and n = ord(b).