CBSE NCERT Solutions Chapter 5 Arithmetic Progressions Exercise 5.3 Question 9
9. If the sum of first 7 terms of an AP is 49 and that of 17 terms is 289, find the sum of first n terms.
Solution:
It is given that sum of first 7 terms of an AP is equal to 49 and sum of first 17 terms is equal to 289.
Applying formula,
to find sum of n terms of AP , we get
(1)
And,
Putting equation (1) in the above equation, we get
Putting value of
in equation (1), we get
Again applying formula,
to find sum of n terms of AP , we get
Therefore, sum of n terms of AP is equal to
.
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