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# Revision Notes on Combinations

• If certain objects are to be arranged in such a way that the order of objects is not important, then the concept of combinations is used.

• The number of combinations of n things taken r (0 < r < n) at a time is given by nCr= n!/r!(n-r)!

• The relationship between combinations and permutations is nCr = nPr/r!

• The number of ways of selecting r objects from n different objects subject to certain condition like:

1. k particular objects are always included =  n-kCr-k

2. k particular objects are never included =  n-kCr

• The number of arrangement of n distinct objects taken r at a time so that k particular objects are

1. Always included = n-kCr-k.r!,

2. Never included = n-kCr.r!.

• In order to compute the combination of n distinct items taken r at a time wherein, the chances of occurrence of any item are not fixed and may be one, twice, thrice, …. up to r times is given by n+r-1Cr

• If there are m men and n women (m > n) and they have to be seated or accommodated in a row in such a way that no two women sit together then total no. of such arrangements

= m+1Cn. m! This is also termed as the Gap Method.

• If there is a problem that requires n number of persons to be accommodated in such a way that a fixed number say ‘p’ are always together, then that particular set of p persons should be treated as one person. Hence, the total number of people in such a case becomes (n-m+1). Therefore, the total number of possible arrangements is (n-m+1)! m! This is also termed as the String Method.

• Let there be n types of objects with each type containing at least r objects. Then the number of ways of arranging r objects in a row is nr.

• The number of selections from n different objects, taking at least one

= nC1 + nC2 + nC3 + ... + nCn = 2n - 1.

• Total number of selections of zero or more objects from n identical objects is n+1.

• Selection when both identical and distinct objects are present:

• The number of selections, taking at least one out of a1 + a2 + a3 + ... an + k objects, where a1 are alike (of one kind), a2 are alike (of second kind) and so on ... an are alike (of nth kind), and k are distinct

{[(a1 + 1)(a2 + 1)(a3 + 1) ... (an + 1)]2k} - 1.

• Combination of n different things taken some or all of n things at a time is given by 2n – 1.

• Combination of n things taken some or all at a time when p of the things are alike of one kind, q of the things are alike and of another kind and r of the things are alike of a third kind

= [(p + 1) (q + 1)(r + 1)….] – 1

• Combination of selecting s1 things from a set of n1 objects and s2 things from a set of n2 objects where combination of s1 things and s2 things are independent is given by n1Cs1 x n2Cs2

• Some results related to nCr

1. nCr = nCn-r

2. If nCr = nCk, then r = k or n-r = k

3. nCr + nCr-1 = n+1Cr

4. nCr = n/r  n-1Cr-1

5. nCr/nCr-1 = (n-r+1)/ r

6. If n is even nCr is greatest for r = n/2

7. If n is odd, is greatest for r = (n-1) /2, (n+1)/2