# NCERT Solutions for Class 11th Mathematics

## Chapter 9. Sequences and Series

### Exercise 9.1 (Available)

**Exercise 9.1**

Write the first five terms of each of the sequences in Exercises 1 to 6 whose *n*^{th} terms are:

Q1. Write the first five terms of the sequences whose n^{th} term is *a _{n} = n(n+2).*

*Solution:**Q1.*

*is a _{n} = n(n+2)*

*Putting n = 1, 2, 3, 4 and 5 we obtain*

*a _{1} = 1(1 + 2) = 1× 3 = 3 *

*a _{2} = 2(2 + 2) = 2× 4 = 8 *

*a _{3} = 3(3 + 2) = 3× 5 = 15 *

*a _{4} = 4(4 + 2) = 4× 6 = 24 *

*a _{5} = 5(5 + 2) = 5× 7 = 35 *

Therefore, the five terms are 3, 8, 15, 24 and 35

*Q5. *Write the first five terms of the sequences whose n^{th} term is *a _{n} = (-1)^{n-1}5^{n+1}*

*Solution: Q5. *

*a _{n} = (-1)^{n-1}5^{n+1}*

Putting n = 1, 2, 3, 4 and 5 we obtain

*a _{1} = (-1)^{1-1}5^{1+1} = (1)(5)^{2} = 25*

*a _{2} = (-1)^{2-1}5^{2+1} = (-1) (5)^{3} = - 125*

*a _{3} = (-1)^{3-1}5^{3+1} = (1) (5)^{4} = 625*

*a _{4} = (-1)^{4-1}5^{4+1} = (-1) (5)^{5} = - 3125*

*a _{5} = (-1)^{5-1}5^{5+1} = (1) (5)^{6} =156 25*

Thus the five terms are 25, -125, 625, - 3125 and 15625