Sequence and Series | Quantitative Aptitude for CA Foundation PDF Download

Sequence

  • A sequence refers to an ordered list of items, numbers, letters, symbols, or events that follow a specific pattern or rule.
  • The objective is to identify the underlying rule or pattern to predict the next item(s) in the sequence or determine the missing element(s).

Sequence and Series | Quantitative Aptitude for CA Foundation

Let us consider the following collection of numbers-

(1) 28 , 2, 25, 27, ————————

(2) 2 , 7, 11, 19, 31, 51, —————

(3) 1, 2, 3, 4, 5, 6, ———————

(4) 20, 18, 16, 14, 12, 10, —————

  • In (1) the nos. are not arranged in a particular order. In (2) the nos. are in ascending order but they do not obey any rule or law. It is, therefore, not possible to indicate the number next to 51.
  • In (3) we find that by adding 1 to any number, we get the next one. Here the number next to 6 is (6 + 1 = ) 7.
  • In (4) if we subtract 2 from any number we get the nos. that follows. Here the number next to 10 is (10 –2 =) 8.
  • Under these circumstances, we say, the numbers in the collections (1) and (2) do not form sequences whereas the numbers in the collections (3) & (4) form sequences.
  • Thus a sequence may be defined as follows:—
    An ordered collection of numbers a1, a2, a3, a4, ................., an, ................. is a sequence if according to some definite rule or law, there is a definite value of an, called the term or element of the sequence, corresponding to any value of the natural number n.
  • Clearly, a1 is the 1st term of the sequence , a2 is the 2nd term, ................., an is the nth term.
  • In the nth term an , by putting n = 1, 2 ,3 ,......... successively , we get a1, a2 , a3 , a4, .........

Thus it is clear that the nth term of a sequence is a function of the positive integer n. The nth term is also called the general term of the sequence. To specify a sequence, nth term must be known, otherwise it may lead to confusion. A sequence may be finite or infinite.

1. Finite sequence: If the number of elements in a sequence is finite, the sequence is called finite sequence
2. Infinite sequence: If the number of elements is unending, the sequence is infinite.

A finite sequence a1, a2, a3, a4, ................., an is denoted by Sequence and Series | Quantitative Aptitude for CA Foundationand an infinite sequence a1, a2, a3, a4, ................., an ,................. is denoted by Sequence and Series | Quantitative Aptitude for CA Foundation or simply by { an } where an is the nth element of the sequence.

Examples of infinite sequences:

1) The sequence { 1/n } is 1, 1/2, 1/3, 1/4……

2) The sequence { ( – 1 ) n n } is –1, 2, –3, 4, –5,…..

3) The sequence { n } is 1, 2, 3,…

4) The sequence { n / (n + 1) } is 1/2, 2/3, 3/4, 4/5 …….

5) A sequence of even positive integers is 2, 4, 6, .....................................

6) A sequence of odd positive integers is 1, 3, 5, 7, .....................................

Examples of finite sequences:

1) A sequence of even positive integers within 12 i.e., is 2, 4, 6, 10.
2) A sequence of odd positive integers within 11 i.e., is 1, 3, 5, 7, 9. etc.

Question for Sequence and Series
Try yourself:
What is the next number in the sequence 3, 6, 9, 12, ...?
View Solution

Series

  • An expression of the form a1 + a2 + a3 + ….. + an + ............................ which is the sum of the elements of the sequence { an } is called a series
  • If the series contains a finite number of elements, it is called a finite series, otherwise called an infinite series.
  • If Sn = u1 + u2 + u3 + u4 + ……. + un, then Sn is called the sum to n terms(or the sum of the first n terms) of the series and the term sum is denoted by the Greek letter Σ. Thus, Sequence and Series | Quantitative Aptitude for CA Foundation or simply by Σun.

Sequence and Series | Quantitative Aptitude for CA Foundation

Examples:

(i) 1 + 3 + 5 + 7 + ............................ is a series in which 1st term = 1, 2nd term = 3 , and so on.

(ii) 2 – 4 + 8 –16 + ..................... is also a series in which 1st term = 2, 2nd term = –4 , and so on.

Arithmetic Progression (A.P.)

  • A sequence a1, a2 ,a3, ……, an is called an Arithmetic Progression(A.P.) when a2 – a1 = a3 – a= ….. = an – an–1
  • That means A. P. is a sequence in which each term is obtained by adding a constant d to the preceding term
  • This constant ‘d’ is called the common differenceof the A.P. 
  • If 3 numbers a, b, c are in A.P., we say b – a = c – b or a + c = 2b; b is called the arithmetic mean between a and c.

Examples: 
1) 2,5,8,11,14,17,…… is an A.P. in which d = 3 is the common difference.

2) 15,13,11,9,7,5,3,1,–1, is an A.P. in which –2 is the common difference.

Explanation: In (1) 2nd term = 5 , 1st term = 2, 3rd term = 8,

so 2nd term – 1st term = 5 – 2 = 3, 3rd term – 2nd term = 8 – 5 = 3

Here the difference between a term and the preceding term is same that is always constant.

This constant is called common difference.

Now in generel an A.P. series can be written as a, a + d, a + 2d, a + 3d, a + 4d, ……

where ‘a’ is the 1st term and ‘d’ is the common difference.

Thus 1st term ( t1 ) = a = a + ( 1 – 1 ) d

2nd term ( t2 ) = a + d = a + ( 2 – 1 ) d

3rd term (t3) = a + 2d = a + (3 – 1) d

4th term (t4) = a + 3d = a + (4 – 1) d

…………………………………………….

nth term (tn) = a + ( n – 1) d, where n is the position number of the term.

Using this formula we can get

50th term (= t50) = a+ (50 – 1) d = a + 49d

Solved Examples

Example 1: Find the 7th term of the A.P. 8, 5, 2, –1, –4,…..
Sol: Here a = 8, d = 5 – 8 = –3

Now t7 = 8 + (7 – 1) d

= 8 + (7 – 1) (– 3)

= 8 + 6 (– 3)

= 8 – 18

= – 10
Example 2: Which term of the APSequence and Series | Quantitative Aptitude for CA Foundation
Sol:
Sequence and Series | Quantitative Aptitude for CA Foundation
We may write
Sequence and Series | Quantitative Aptitude for CA Foundation
or, 17 = 3 + ( n – 1)

or, n = 17 – 2 = 15
Hence, 15th term of the A.P. is Sequence and Series | Quantitative Aptitude for CA Foundation
Example 3: If 5th and 12th terms of an A.P. are 14 and 35 respectively, find the A.P.
Sol:Let a be the first term & d be the common difference of A.P.

t5 = a + 4d = 14

t12 = a + 11d = 35

On solving the above two equations,

7d = 21 = i.e., d = 3

and a = 14 – (4 × 3) = 14 – 12 = 2

Hence, the required A.P. is 2, 5, 8, 11, 14,……………
Example 4: Divide 69 into three parts which are in A.P. and are such that the product of the first two parts is 483.
Sol: Given that the three parts are in A.P., let the three parts which are in A.P. be a – d, a, a + d.........

Thus a – d + a + a + d = 69

or 3a = 69

or a = 23

So the three parts are 23 – d, 23, 23 + d

Since the product of first two parts is 483, therefore, we have

23 ( 23 – d ) = 483

or 23 – d = 483 / 23 = 21

or d = 23 – 21 = 2

Hence, the three parts which are in A.P. are

23 – 2 = 21, 23, 23 + 2 = 25

Hence the three parts are 21, 23, 25.

Example 5: Find the arithmetic mean between 4 and 10.
Sol: We know that the A.M. of a & b is = ( a + b ) /2

Hence, The A. M between 4 & 10 = ( 4 + 10 ) /2 = 7
Example 6: Insert 4 arithmetic means between 4 and 324.
4, –, –, –, –, 324

Sol: Here a= 4, d = ? n = 2 + 4 = 6, tn = 324

Now tn = a + ( n – 1 ) d

or 324= 4 + ( 6 – 1 ) d

or 320= 5d i.e., = i.e., d = 320 / 5 = 64

So the 1st AM = 4 + 64 = 68

2nd AM = 68 + 64 = 132

3rd AM = 132 + 64 = 196

4th AM = 196 + 64 = 260

Sum of the first n terms

Let S be the Sum, a be the 1st term and ℓ the last term of an A.P. If the number of term are n,

then tn = ℓ. Let d be the common difference of the A.P.

Now S = a + ( a + d ) + ( a + 2d ) + .. + ( ℓ – 2d ) + ( ℓ – d ) + ℓ

Again S = ℓ + ( ℓ – d ) + ( ℓ – 2d ) + …. + ( a + 2d ) + ( a + d ) + a

On adding the above, we have

2S = ( a + ℓ ) + ( a + ℓ ) + ( a + ℓ ) + …… + ( a + ℓ )

= n( a + ℓ )

or S = n( a + ℓ ) / 2

Note: The above formula may be used to determine the sum of n terms of an A.P. when the

first term a and the last term is given.

Now ℓ = tn = a + ( n – 1 ) d
Sequence and Series | Quantitative Aptitude for CA Foundation
or
Sequence and Series | Quantitative Aptitude for CA Foundation

Note: The above formula may be used when the first term a, common difference d and the number of terms of an A.P. are given.

Sum of 1st n natural or counting numbers

S = 1 + 2 + 3 + ……. +……. ( n – 2 ) + ( n – 1 ) + n

Again S = n + ( n – 1 ) + ( n – 2 ) + ……… + 3 + 2 + 1

On adding the above, we get

2S = ( n + 1 ) + ( n + 1 ) +....... to n terms

or 2S = n ( n + 1 )

S = n( n + 1 )/2

Then Sum of first, n natural number is n( n + 1 ) / 2
Sequence and Series | Quantitative Aptitude for CA Foundation
Sum of 1st n odd number

S = 1 + 3 + 5 + …… + ( 2n – 1 )

Sum of first n odd number

S = 1 + 3 + 5 + …… + ( 2n – 1 )

Since S = n{ 2a + ( n –1 ) d } / 2, we find
Sequence and Series | Quantitative Aptitude for CA Foundation
or S = n2

Then sum of first, n odd numbers is n2, i.e. 1 + 3 + 5 + ..... + ( 2n – 1 ) = n2

Sum of the Squares of the first, n natural nos.

Let S = 12 + 22 + 32 + …… + n2

We know m3 – ( m – 1 )3 = 3m2 – 3m + 1

We put m = 1, 2, 3,……,n
Sequence and Series | Quantitative Aptitude for CA Foundation
Adding both sides term by term
Sequence and Series | Quantitative Aptitude for CA Foundation
Thus sum of the squares of the first, n natural numbers is Sequence and Series | Quantitative Aptitude for CA Foundation
Sequence and Series | Quantitative Aptitude for CA Foundation
Similarly, sum of the cubes of first n natural number can be found out as Sequence and Series | Quantitative Aptitude for CA Foundation

by taking the identity

m4 – ( m – 1 )4 = 4m3 – 6m2 + 4m – 1 and putting m = 1, 2, 3,…., n.

Thus

Sequence and Series | Quantitative Aptitude for CA Foundation

Question for Sequence and Series
Try yourself:
Find the 10th term of the Arithmetic Progression 3, 6, 9, 12, ...
View Solution

Geometric Progression (G.P.)

  • If in a sequence of terms each term is a constant multiple of the preceding term, then the sequence is called a Geometric Progression (G.P).
  • The constant multiplier is called the common ratio.

Examples

1) In 5, 15, 45, 135,….. common ratio is 15/5 = 3

2) In 1, 1/2, 1/4, 1/9 … common ratio is (1/2) /1 = 1/2

3) In 2, –6, 18, –54, …. common ratio is (–6) / 2 = –3

Illustrations: Consider the following series :–

(i) 1 + 4 + 16 + 64 + …………….

Here second term / first term = 4/1 = 4; third term / second term = 16/4 = 4; fourth term/third term = 64/16 = 4 and so on. Thus, we find that, in the entire series, the ratio of any term and the term preceding it, is a constant.

(ii) 1/3 – 1/9 + 1/27 – 1/81 + ………….

Here second term / 1st term = (–1/9) / ( 1/3) = –1/3

third term / second term = ( 1/27 ) / ( –1/9 ) = –1/3

fourth term / third term = ( –1/81 ) / (1/27 ) = –1/3 and so on.

Here also, in the entire series, the ratio of any term and the term preceding one is constant.

The above-mentioned series are known as Geometric Series.

Let us consider the sequence a, ar, ar2, ar3, ….

1st term = a, 2nd term = ar = ar2–1, 3rd term = ar2 = ar3–1, 4th term = ar3 = ar4 –1, …..

Similarly
nth term of GP tn = arn–1

Thus, common ratio
Sequence and Series | Quantitative Aptitude for CA Foundation
Thus, general term of a G.P is given by arn–1 and the general form of G.P. is
Sequence and Series | Quantitative Aptitude for CA Foundation
Sequence and Series | Quantitative Aptitude for CA Foundation

Solved Examples

Example 1: If Sequence and Series | Quantitative Aptitude for CA Foundationare in G.P. Find the common ratio.
Sol:1st term = a, 2nd term = ar

Ratio of any term to its preceding term = ar/a = r = common ratio.

Example 2: Which term of the progression 1, 2, 4, 8,… is 256?
Sol: a = 1, r = 2/1 = 2, n = ? tn = 256

tn = arn–1

or 256 = 1 × 2n–1 i.e., 28 = 2n–1 or, n – 1 = 8 i.e., n = 9

Thus 9th term of the G. P. is 256

Geometric Mean

  • If a, b, c are in G.P we get b/a = c/b => b2 = ac, bis called the geometric mean between a and c.

Example 1: Insert 3 geometric means between 1/9 and 9.
Sol:
Sequence and Series | Quantitative Aptitude for CA Foundation
Sequence and Series | Quantitative Aptitude for CA Foundation

Sequence and Series | Quantitative Aptitude for CA Foundation

Example 2: Find the G.P where 4th term is 8 and 8th term is 128/625.
Sol:
Let a be the 1st term and r be the common ratio.

By the question t4 = 8 and t8 = 128/625

So ar3 = 8 and ar7 = 128 / 625
Therefore
Sequence and Series | Quantitative Aptitude for CA Foundation
Sequence and Series | Quantitative Aptitude for CA Foundation
Now, Sequence and Series | Quantitative Aptitude for CA Foundation

Thus the G. P is

125, 50, 20, 8, 16/5, ………..

When r = –2/5 , a = –125 and the G.P is –125, 50, –20, 8, –16/5 ,………

Finally, the G.P. is 125, 50, 20, 8, 16/5, ………..

or, –125, 50, –20, 8, –16/5,………

Sum of first n terms of a G.P.

Let a be the first term and r be the common ratio. So the first n terms are a, ar, ar2, …... arn–1.

If S be the sum of n terms,
Sequence and Series | Quantitative Aptitude for CA Foundation
Subtracting (i) from (ii)
or
Sequence and Series | Quantitative Aptitude for CA Foundation

If r = 1 , then Sn = a + a + a+ ……….. to n terms = na

If the nth term of the G. P be l then Sequence and Series | Quantitative Aptitude for CA Foundation
Thus, Sequence and Series | Quantitative Aptitude for CA Foundation

So, when the last term of the G. P is known, we use this formula.

Sum of infinite geometric series

S = a ( 1 – rn ) / (1 – r) when r < 1

= a (1 – 1/Rn) / ( 1 – 1/R ) (since r < 1 , we take r = 1/R).
Sequence and Series | Quantitative Aptitude for CA Foundation
Thus
Sequence and Series | Quantitative Aptitude for CA Foundation
i.e. Sum of G.P. upto infinity is Sequence and Series | Quantitative Aptitude for CA Foundation
Sequence and Series | Quantitative Aptitude for CA Foundation

Solved Examples

Example 1: Find the sum of 1 + 2 + 4 + 8 + … to 8 terms.
Sol:
Here a = 1, r = 2/1 = 2 , n = 8

Let S = 1 + 2 + 4 + 8 + …… to 8 terms

= 1 ( 28 – 1 ) / ( 2 – 1 ) = 28 – 1 = 255

Example 2: Find the sum to n terms of 6 + 27 + 128 + 629 + …….
Sol:
Required Sum = ( 5 + 1 ) + (52 + 2 ) + ( 53 +3 ) + ( 54 + 4 ) + … to n terms

= ( 5 + 52 +53 + …… + 5n ) + ( 1 + 2 + 3 + .. + n terms)

= {5 ( 5n – 1 ) / (5 – 1 )} + {n ( n + 1 ) / 2}

= {5 ( 5n – 1 ) /4} + {n ( n + 1 ) / 2}

Example 3: Find the sum to n terms of the series 3 + 33 + 333 + …….
Sol:
Let S denote the required sum.

i.e. S = 3 + 33 + 333 + ………….. to n terms

= 3 (1 + 11 + 111 + ……. to n terms)
Sequence and Series | Quantitative Aptitude for CA Foundation
Sequence and Series | Quantitative Aptitude for CA Foundation
Sequence and Series | Quantitative Aptitude for CA Foundation
Sequence and Series | Quantitative Aptitude for CA Foundation

Example 4: Find the sum of n terms of the series 0.7 + 0.77 + 0.777 + …. to n terms.
Sol: Let S denote the required sum.

i.e. S = 0.7 + 0.77 + 0.777 + ….. to n terms

= 7 (0.1 + 0.11 + 0.111 + …. to n terms)
Sequence and Series | Quantitative Aptitude for CA Foundation
Sequence and Series | Quantitative Aptitude for CA Foundation
So,
Sequence and Series | Quantitative Aptitude for CA Foundation

Example 5: Evaluate Sequence and Series | Quantitative Aptitude for CA Foundation using the sum of an infinite geometric series.
Sol: 
Sequence and Series | Quantitative Aptitude for CA Foundation

Sequence and Series | Quantitative Aptitude for CA Foundation
Sequence and Series | Quantitative Aptitude for CA Foundation

Example 6: Find three numbers in G. P whose sum is 19 and product is 216.
Sol: Let the 3 numbers be a/r, a, ar.

According to the question a/r × a × ar = 216

or a3 = 63 => a =6

So the numbers are 6/r, 6, 6r

Again 6/r + 6 + 6r = 19
Sequence and Series | Quantitative Aptitude for CA Foundation
Sequence and Series | Quantitative Aptitude for CA Foundation
So the numbers are
Sequence and Series | Quantitative Aptitude for CA Foundation

Example 7: A person is employed in a company at ₹3000 per month and he would get an increase of ₹100 per year. Find the total amount which he receives in 25 years and the monthly salary in the last year.
Sol: 
He gets in the 1st year at the rate of 3000 per month;

In the 2nd year he gets at the rate of ₹ 3100 per month;

In the 3rd year at the rate of ₹ 3200 per month so on.

In the last year the monthly salary will be

₹{3000 + ( 25 – 1 ) × 100} = ₹ 5400
Total amount = Sequence and Series | Quantitative Aptitude for CA Foundation
Sequence and Series | Quantitative Aptitude for CA Foundation

(II) A person borrows ` 8,000 at 2.76% Simple Interest per annum. The principal and the

interest are to be paid in the 10 monthly instalments. If each instalment is double the

preceding one, find the value of the first and the last instalment.

SOLUTION:

Interest to be paid = 2.76 × 10 × 8000 / 100 × 12 = ₹184

Total amount to be paid in 10 monthly instalment is ` (8000 + 184) = ₹8184

The instalments form a G P with common ratio 2 and so ` 8184 = a (210 – 1 ) / ( 2 – 1 ),

a = 1st instalment

Here a = ₹ 8184 / 1023 = ₹ 8

The last instalment = ar10—1 = 8 × 29 = 8 × 512 = ₹4096


Summary

⇒ Sequence: An ordered collection of numbers a1, a2, a3, a4, ................., an, ................. is a sequence if according to some definite rule or law, there is a definite value of an, called the term or element of the sequence, corresponding to any value of the natural number n.

Series: An expression of the form a1 + a2 + a3 + ….. + an + ............................ which is the sum of the elements of the sequence { an } is called a series. If the series contains a finite number of elements, it is called a finite series, otherwise called an infinite series.

Arithmetic Progression: A sequence a1, a2 ,a3, ……, an is called an Arithmetic Progression (A.P.) when a2 – a, = a3 – a2 = ….. = an – an–1. That means A. P. is a sequence in which each term is obtained by adding a constant d to the preceding term. This constant ‘d’ is called the common difference of the A.P. If 3 numbers a, b, c are in A.P., we say b – a = c – b or a + c = 2b; b is called the arithmetic mean between a and c.

nth term ( tn ) = a + ( n – 1 ) d,

Where a = First Term

D = Common difference= tn- tn-1

Sum of n terms of AP=Sequence and Series | Quantitative Aptitude for CA Foundation

⇒ Sum of the first n terms: Sum of 1st n natural or counting numbers

S = n( n + 1 )/2

Sum of 1st n odd number : S = n2

Sum of the Squares of the first, n natural numbers:
Sequence and Series | Quantitative Aptitude for CA Foundation
sum of the squares of the first, n natural numbers is
Sequence and Series | Quantitative Aptitude for CA Foundation

⇒ Geometric Progression (G.P). If in a sequence of terms each term is constant multiple of the proceeding term, then the sequence is called a Geometric Progression (G.P). The constant multiplier is called the common ratio
Sequence and Series | Quantitative Aptitude for CA Foundation
Sum of first n terms of a G P:
Sequence and Series | Quantitative Aptitude for CA Foundation
Sum of infinite geometric series
Sequence and Series | Quantitative Aptitude for CA Foundation

⇒ A.M. of a & b is = ( a + b ) /2

⇒ If a, b, c are in G.P we get b/a = c/b => b2= ac, b is called the geometric mean between a and c.

Question for Sequence and Series
Try yourself:
Which term of the geometric progression 2, 6, 18, 54, ... is 1458?
View Solution

The document Sequence and Series | Quantitative Aptitude for CA Foundation is a part of the CA Foundation Course Quantitative Aptitude for CA Foundation.
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FAQs on Sequence and Series - Quantitative Aptitude for CA Foundation

1. What is the difference between a sequence and a series?
Ans. A sequence is a list of numbers arranged in a specific order, where each number is called a term. A series, on the other hand, is the sum of the terms of a sequence. For example, in the sequence 1, 2, 3, the series would be 1 + 2 + 3 = 6.
2. How do you identify an arithmetic progression (A.P.)?
Ans. An arithmetic progression (A.P.) is identified by a constant difference between consecutive terms. For example, in the sequence 2, 5, 8, 11, the common difference is 3. If you can find a fixed number that is added to each term to get the next term, it is an A.P.
3. What is the formula for the nth term of an arithmetic progression?
Ans. The formula for the nth term (Tn) of an arithmetic progression is given by Tn = a + (n - 1)d, where 'a' is the first term, 'n' is the term number, and 'd' is the common difference.
4. How is a geometric progression (G.P.) defined?
Ans. A geometric progression (G.P.) is defined as a sequence where each term after the first is found by multiplying the previous term by a fixed non-zero number called the common ratio. For example, in the sequence 3, 6, 12, 24, the common ratio is 2.
5. What is the geometric mean of two numbers?
Ans. The geometric mean of two positive numbers a and b is calculated using the formula √(a * b). It represents the central tendency of the two numbers in a multiplicative sense and is particularly useful in contexts involving growth rates and proportional relationships.
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