**CBSE NCERT Solutions Chapter 5 Arithmetic Progressions Exercise 5.3 Question 4**

**4. How many terms of the AP: 9, 17, 25, ... must be taken to give a sum of 636?**

**Solution:**

First term = a = 9

Common difference = d = 17 -9 = 8

**Applying formula**, to find sum of n terms of AP, **we get**

It is a quadratic equation. We can solve it using different methods. Lets solve it using quadratic formula.

**Comparing equation** with general form , **we get**

and

**Applying** quadratic formula, **and putting values of a, b and c, we get**

We discard negative value of here because n cannot be in negative, n can only be a positive integer.

Therefore, .

**Therefore, 12 terms of the given sequence make sum equal to 636.**