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,
lcm
Therefore, We may count the number of elements in with the property that ,
lcm .
Clearly, this requires either and or and .
Case:1 and
Here there are 1 choice for and 6 choices for ( namely 1,2,4,5,7 and 8).
This gives element of order 9.
Case 2. and
This time 2 choices for (namely, 1, and 2) and as before 6 choices for .
This gives elements.
Therefore the total number of elements of order 9 is 6+12=18.
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