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+17 votes

Match the pairs in the following questions by writing the corresponding letters only.

A. The number distinct binary trees with $n$ nodes. P. $\frac{n!}{2}$

B. The number of binary strings of length of $2n$ with
an equal number of $0’s$ and $1’s$.

Q. $ \binom{3n}{n}$
C. The number of even permutation of $n$ objects. R. $\binom{2n}{n}$
D. The number of binary strings of length $6n$ which are
palindromes with $2n$  $0’s$.
S. $\frac{1}{1+n}\binom{2n}{n}$
asked in Combinatory by Veteran (59.6k points)
edited by | 871 views

2 Answers

+22 votes
Best answer
  1. - S Catalan number http://
  2. - R Choosing $n$ locations for $0$'s out of $2n$ locations. The remaining $n$ locations are filled with $1$'s (no selection required).
  3. - P An even permutation is a permutation obtainable from an even number of two-element swaps, For a set of $n$ elements and $n>2$, there are $n!/2$ even permutations.
    Ref -
  4.  - Q 

Length $= 6n$, as it is palindrome, we need to select only the first half part of the string.

Total length to consider is $3n$ (Remaining $3n$ will be revese of this $3n$)

Now, choose $n \ 0's$ out of $3n$. So Q is correct for D.

answered by Boss (43k points)
edited by
Explain even permutations with an example
+2 votes

A - S
B - R
C - P
D - Q
answered by Boss (34k points)

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