Suppose 32 objects are placed along a circle at equal distances. In how many ways can 3 objects be chosen from among them so that no two of three chosen objects are adjacent nor diametrically opposite?
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Arrange all things in a straight line a 1 , a 2 . . . … . a 3 2
See the following arrangement
P a Q b R c S
Here a, b, c are any three things P, Q, R, S are no of things left in their middle and aside.
Here P + Q + R + S = 2 9 where P , S ≥ 0 & Q , R ≥ 1
So the no. of solutions of this equation are 2 7 + 4 − 1 C 3 = 4 0 6 0 . Now here we had taken them in a line.
When arranged in a circle a 1 and a 3 2 can't be together so we will remove the cases in which both of them are together, there are 28 cases to be removed.
So now we get 4060-28 = 4032 cases
Now we have to remove the cases of diametrically opposite things.
Here two things a p ; a q p>q are diametrically opposite if p − q = 1 6 we will get 16 pairs with 26 arrangements corresponding to every pair so total cases are 26×16=416
Finally the answer comes out to be 4032-416=3616
Total cases are 3 2 c 3 = 4 9 6 0
Cases to be subtracted:
1) All 3 adjacent = 32
2) 2 adjacent and third one not adjacent to both= 32*28 = 896
3) 2 diametrically opposite and third one not adjacent to both = 16*26 = 416
So, answer is 4 9 6 0 − 3 2 − 8 9 6 − 4 1 6 = 3 6 1 6 .
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There are 3 2 C 3 = 4 9 6 0 triples altogether.
Of these, 1 6 × 3 0 = 4 8 0 contain diametrically opposite points ( 1 6 choices for the diametrically opposite points, and 3 0 choices for the other point).
A total of 3 2 × 2 8 + 3 2 = 3 2 × 2 9 = 9 2 8 triples contain adjacent points ( 3 2 × 2 8 of these triples have just two consecutive points, while the other 3 2 have three consecutive points).
A total of 1 6 × 4 = 6 4 triples contain adjacent points and diametrically opposite points.
Thus 4 9 6 0 − 4 8 0 − 9 2 8 + 6 4 = 3 6 1 6 triples contain neither adjacent points nor diametrically opposite points.