CLASS_11_MATHS_SOLUTIONS_NCERT

Class XI Chapter 9 – Sequences and Series Maths
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A5 a 5d
8 53 815 23
Thus, the required five numbers between 8 and 26 are 11, 14, 17, 20 and 23.
Question 15:
n n
a b
If is the A.M. between a and b, then find the value of n.
n1 n1
a b
Solution 15:
A.M. of a and b
a
b
2
According to the given condition,
n
a b a b
2 a b
n
n1 n1
n1 n1
n n
a ba b 2a b
a ab ba b 2a 2b
n n1 n1
n n n
ab a b a b
n1 n1
n n
n1 n n n1
ab b a a b
n
b
n1 1
b a b a a b
a
n1 n1
n1 0
a a
1
b b
n 1
0
n
1
Question 16:
Between 1 and 31, m numbers have been inserted in such a way that the resulting sequence is an
A.P. and the ratio of 7 th and numbers is 5:9. Find the value of m.
Solution 16:
m 1 th
Let A1, A2,..... A
m
be m numbers such that 1, A1, A2,..... A
m,31
is an A.P.
Here, a 1, b 31, n m 2
m
d
m
d
31 1 2 1
30 1
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