 # Sequences and Series Class 11 Notes- Chapter 9

Let us assume that there is a generation gap of 25 years and we are required to find total ancestors that a person might have over 400 years. Here, the total generations = 400/25 = 16. The number of ancestors for the 1st, 2nd, 3rd, . . . 16th generations form what we call a sequence. The different numbers occurring in a sequence are known as its terms denoted by m1, m2, m3, . . . . . , mn, . . . . . etc.. [Here, the position of the term is denoted by the subscript].

A sequence m1, m2, m3, . . . . , mn is said to be an arithmetic sequence or progression if

$$\begin{array}{l}m_{n+1}=m_{n}+d\end{array}$$
, n ∈ N, where the 1st term and the common difference of an A.P are denoted by m1 and d respectively.

Also Refer:  Sequence and Series

Let us consider an Arithmetic Progression with p as the 1st term and d as the common difference, i.e., p, p + d, p + 2d, . . . . Then the pth term of the A.P. is given by pn = p + (n – 1)d. If a constant term is added, subtracted, multiplied, or divided to an Arithmetic Progression then the resulting sequence is also an Arithmetic Progression. Given two numbers p and q. A number A can be inserted between them such that p, A, q is an AP. Such number is known as the A.M (arithmetic mean) of the numbers p and q. The sum of the 1st n terms of an A.P (Arithmetic Progression) is calculated by

$$\begin{array}{l}S_{n}=\frac{n}{2}\left [ 2a\;+\;(n-1)d \right ]\end{array}$$

The 1st term of a G.P is denoted by ‘a’ and the common ratio by ‘r’. The general term of a Geometric Progression is given by

$$\begin{array}{l}a_{n}=ar^{n-1}\end{array}$$
and the sum of the 1st n terms is given by,

$$\begin{array}{l}S_{n}=\frac{a(r^{n}-1)}{r-1}\;\;or\;\;\frac{a(1-r^{n})}{1-r}\end{array}$$

The geometric mean of any two +ve numbers p and q is given by G =

$$\begin{array}{l}\sqrt{pq}\end{array}$$
and the sequence p, G, q is also a G.P.

To get more details on geometric mean, visit here.

### Sequences and Series Class 11 Practice Problems

1. If Geometric and Arithmetic Mean of two +ve numbers p and q are 12 and 10 respectively, find both the numbers.
2. The fourth term of a Geometric Progression (G.P.) is square of its 2nd term and its 1st term is -6. Find its 6th term.
3. Determine the sum of n terms of the sequence, 7, 77, 777, 7777, . . . . .
4. Determine the sum of n terms of the series: 3 + 9 + 17 + 27 + 39 + . . . .
5. Find the 24th term of a sequence defined by pn = (p – 1) (3 + p) (2 – p)?

Also Refer: Sequence and Series Formulas

## Frequently asked Questions on CBSE Class 11 Maths Notes Chapter 9: Sequences and Series

### What is a ‘Sequence’?

It is an arrangement of two or more things in a successive order.

### What is a ‘Series’?

A series is defined as the sum of the elements of a sequence.

### What is a ‘Natural’ number?

A natural number is a number that occurs commonly and obviously in nature.