**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 .**