CLASS_11_MATHS_SOLUTIONS_NCERT

Class XI Chapter 9 – Sequences and Series Maths
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Exercise 9.2
Question 1:
Find the sum of odd integers from 1 to 2001.
Solution 1:
The odd integers from 1 to 2001 are 1, 3, 5 …… 1999, 2001.
This sequence forms an A.P.
Here, first term,
Common difference, d 2
Here,
a 1
a n 1 d 2001
1 n 1 2 2001
2n
2 2000
n
1001
n
Sn
2a n 1
d
2
S 1001 2 1 1001 1 2
n
2
1001 2 1000 2
2
10011001
1002001
Thus, the sum of odd numbers from 1 to 2001 is 1002001.
Question 2:
Find the sum of all natural numbers lying between 100 and 1000, which are multiples of 5.
Solution 2:
The natural numbers lying between 100 and 1000, which are multiples of 5, are 105, 110, ….
995.
Here,
a 105
and
a n 1 d 995
d 5
105 n 1 5 995
n 1 5 995 105 890
n1
178
n
179
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