We know that
The order of an element in a direct product of finite number of finite groups is the least common multiple of the order of components of the element. In symbols,
"|(g_1,g_2,.........,g_n)|=" lcm "(|g_1|,|g_2|,..........,|g_n|)"
Therefore, We may count the number of elements "(a,b)" in "\\Z_3 \\bigoplus \\Z_9" with the property that ,
"9=|(a,b)|=" lcm "(|a|,|b|)" .
Clearly, this requires either "|a|=1" and "|b|=9" or "|a|=3" and "|b|=9" .
Case:1 "|a|=1" and "|b|=9"
Here there are 1 choice for "a" and 6 choices for "b" ( namely 1,2,4,5,7 and 8).
This gives "1\u00d76=6" element of order 9.
Case 2. "|a|=3" and "|b|=9"
This time 2 choices for "a" (namely, 1, and 2) and as before 6 choices for "b" .
This gives "2\u00d76=12" elements.
Therefore the total number of elements of order 9 is 6+12=18.
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