To solve the problem of finding the number of ways to select 3 elements from 10 elements arranged in a circle such that no two are adjacent, we use a case-based approach:
Case 1: The first element is selected
If the first element is chosen, its adjacent elements (2nd and 10th) are excluded. We need to select 2 more non-adjacent elements from positions 3-9 (7 positions, linear arrangement).
The number of ways to choose 2 non-adjacent elements from 7 linear positions is:
[ \binom{7-2+1}{2} = \binom{6}{2} = 15 ]
Case 2: The first element is not selected
We now choose 3 non-adjacent elements from positions 2-10 (9 linear positions).
The number of ways to choose 3 non-adjacent elements from 9 linear positions is:
[ \binom{9-3+1}{3} = \binom{7}{3} = 35 ]
Total Ways
Add the results of both cases:
[ 15 + 35 = 50 ]
Answer: (\boxed{50})


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