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