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.
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
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 .