CLASS_11_MATHS_SOLUTIONS_NCERT

Class XI Chapter 7 – Permutation and Combination Maths
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Exercise 7.4
Question 1:
n n
If C8 C2
, find n C
2
Solution 1:
It is known that,
Therefore,
n
n
C8 C2 n 8 2 10
n
n
C C a b or m a b
a
10 10! 10! 1098!
n C2 C2
45
2! 10 2 ! 2!8! 218!
b
Question 2:
Determine n if
(i)
2n
C : C 12 :1
n
3 3
Solution 2:
(i)
C
2n
3
n
C
3
12
1
2 n ! 3! n3 ! 12
3! 2n
3 ! n! 1
(ii)
2n
C : C 11:1
n
3 3
2n 2n 1 2n 2 2n 3 ! n 3 !
12
2n 3 ! n n 1 n 2 n 3 !
2 2n1 2n2
n1 n2
n
n
n1n2
4 2 1 1
2n
1
n 2
3
2n1 3 n
2
2n1 3n
6
3n 2n 1
6
n
5
12
12
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