All questions of Series for Electrical Engineering (EE) Exam

After striking the floor, a rubber ball rebounds to 4/5th of the height from which it has fallen. Find the total distance that it travels before coming to rest if it has been gently dropped from a height of 120 metres.
  • a)
    540 m
  • b)
    960 m
  • c)
    1080 m
  • d)
    1020 m
  • e)
    1120 m
Correct answer is option 'C'. Can you explain this answer?

The first drop is 120 metres. After this the ball will rise by 96 metres and fall by 96 metres. This process will continue in the form of infinite GP with common ratio 0.8 and first term 96. The required answer is given by the formula:
a/(1-r) 
Now,
[{120/(1/5)}+{96/(1/5)}] 
= 1080 m.

Find  the  15th  term  of  an  arithmetic  progression  whose  first  term  is  2  and  the  common  difference  is 3
  • a)
    45
  • b)
    38
  • c)
    44
  • d)
    40
Correct answer is option 'C'. Can you explain this answer?

Aisha Gupta answered
Method to Solve :

A ( first term ) :- 2

d ( common difference ) :- 3

n = 15

To find nth term we have formula as

an = a + ( n - 1 )d

a15 = 2 + 14 � 3

a15 = 2 + 42

a15 = 44

How many terms are there in 20, 25, 30......... 140
  • a)
    22
  • b)
    25
  • c)
    23
  • d)
    24
Correct answer is option 'B'. Can you explain this answer?

Anaya Patel answered
Number of terms = { (1st term - last term) / common difference} + 1
= {(140-20) / 5} + 1
⇒ (120/5) + 1
⇒ 24 + 1 = 25.

Find the 15th term of the sequence 20, 15, 10....
  • a)
    -45
  • b)
    -55
  • c)
    -50
  • d)
    0
Correct answer is option 'C'. Can you explain this answer?

Since above sequence is in A.P with common difference of -5 and first term 20.
Then applying formula of AP we get 15 term as
20 + (n-1) d.
15 term is 20 + (15-1) -5 i.e. -50

The sum of 5 numbers in AP is 30 and the sum of their squares is 220. Which of the following is the third term?
  • a)
    5
  • b)
    6
  • c)
    8
  • d)
    9
Correct answer is option 'B'. Can you explain this answer?

Isha Mukherjee answered
Since the sum of 5 numbers in AP is 30, their average would be 6. The average of 5 terms in AP is
also equal to the value of the 3rd term (logic of the middle term of an AP). Hence, the third term’s
value would be 6. Option (b) is correct.

If the nth term of AP is m and the nth term of the same AP is m, then (m + n)th term of AP is
  • a)
    m+n
  • b)
    0
  • c)
    m-n 
  • d)
    -m+n
Correct answer is option 'B'. Can you explain this answer?

  • Tn = m = a + (n - l ) d
  • Tm = n= a + (m - l ) d
  • subtracting them we get d=-1
  • and a=m+n-1
  • Add the two and solve through the options given.
  • then (m + n)th term of AP= a+(m+n-1)d
  • Putting values of a and d in the solution, we get
  •                                         =  m+n-1+(m+n-1)x-1
  •                                         = 0

The sum of the first four terms of an AP is 28 and sum of the first eight terms of the same AP is 88.Find the sum of the first 16 terms of the AP?
  • a)
    346
  • b)
    340
  • c)
    304
  • d)
    268
Correct answer is option 'C'. Can you explain this answer?

Abhay Shah answered
Think like this:
The average of the first 4 terms is 7, while the average of the first 8 terms must be 11.
Now visualize this :
Hence, d = 4/2 = 2 [Note: understand this as a property of an A.P.]
Hence, the average of the 6th and 7th terms = 15 and the average of the 8th and 9th term = 19
But this (19) also represents the average of the 16 term A.P.
Hence, required answer = 16 × 19 = 304.

How many terms are there in the GP 5, 20, 80, 320........... 20480?
  • a)
    5
  • b)
    6
  • c)
    8
  • d)
    9
  • e)
    7
Correct answer is option 'E'. Can you explain this answer?

Sameer Rane answered
Common ratio, r = 20/5 = 4;
Last term or nth term of GP = ar^[n-1].
20480 = 5*[4^(n-1)];
Or, 4^(n-1) = 20480/5 = 4^8;
So, comparing the power, 
Thus, n-1 = 8;
Or, n = 7;
Number of terms = 7.

How many terms are there in the AP 20, 25, 30,… 130.
  • a)
    22
  • b)
    23
  • c)
    21
  • d)
    24
Correct answer is option 'B'. Can you explain this answer?

Arnav Rane answered
In order to count the number of terms in the AP, use the short cut:
[(last term – first term)/ common difference] + 1. In this case it would become:
[(130 – 20)/5] +1 = 23. Option (b) is correct.

A group of friends have some money which was in an increasing GP. The total money with the first and the last friend was Rs 66 and the product of the money that the second friend had and that the last but one friend had was Rs 128. If the total money with all of them together was Rs 126, then how many friends were there?
  • a)
    6
  • b)
    5
  • c)
    3
  • d)
    Cannot be determined
Correct answer is option 'A'. Can you explain this answer?

Prasenjit Basu answered
The sum of money with the first and the last friend = 66. This can be used as a hint. Let us assume the first friend was having Rs 2 and the last friend was having Rs 64. So, the money can be in the sequence 2, 4, 8, 16, 32, 64. It satisfies the given conditions. Alternatively, this can be done by using the formula for tn of GP also.

If a man saves ` 4 more each year than he did the year before and if he saves ` 20 in the first year,after how many years will his savings be more than ` 1000 altogether?
  • a)
    19 years
  • b)
    20 years
  • c)
    21years
  • d)
    18 years
Correct answer is option 'A'. Can you explain this answer?

Aarav Sharma answered
Solution:

To solve this problem, we can use the formula for the sum of an arithmetic progression:

Sn = n/2[2a + (n-1)d]

where Sn is the sum of the first n terms, a is the first term, d is the common difference, and n is the number of terms.

We know that the man saves 4 more each year than he did the year before, so the common difference is d = 4.

We also know that he saves `20 in the first year, so the first term is a = 20.

We want to find out after how many years his savings will be more than `1000 altogether, so we want to find the value of n that satisfies the inequality Sn > 1000.

Substituting the values that we know into the formula, we get:

Sn = n/2[2a + (n-1)d]
Sn = n/2[2(20) + (n-1)(4)]
Sn = n/2[40 + 4n - 4]
Sn = n/2[36 + 4n]
Sn = 18n + 2n^2

Now we can set up the inequality:

Sn > 1000
18n + 2n^2 > 1000
2n^2 + 18n - 1000 > 0

We can solve this quadratic inequality by factoring or using the quadratic formula, but we can also estimate the value of n by looking at the options given.

Option A is 19 years, which means that the man's savings would be:

S19 = 19/2[2(20) + (19-1)(4)]
S19 = 19/2[40 + 72]
S19 = 19/2[112]
S19 = 1064

This is more than `1000, so option A is the correct answer.

We can also check the other options:

Option B: S20 = 20/2[2(20) + (20-1)(4)] = 1120
Option C: S21 = 21/2[2(20) + (21-1)(4)] = 1204
Option D: S18 = 18/2[2(20) + (18-1)(4)] = 988

Therefore, the correct answer is option A, 19 years.

What  is  the  sum  of  the  first  15  terms  of  an  A.P  whose  11 th  and   7 th  terms  are  5.25  and  3.25  respectively
  • a)
    56.25  
  • b)
    60
  • c)
    52.5
  • d)
    none of these
Correct answer is option 'A'. Can you explain this answer?

Ishani Rane answered
a +10d  = 5.25, a+6d  = 3.25,  4d  =  2,  d  =  1/4
a +5  =  5.25, a  = 0.25  = 1/4,   s 15 =  15/2 ( 2 * 1/4 +  14 * 1/4 )
=  15/2 (1/2 +14/2 )     =  15/2 *15/2  =225/ 4   =   56.25

In an arithmetic series consisting of 51 terms, the sum of the first three terms is 65 and the sum of the middle three terms is 129. What is the first term and the common difference of the series?
  • a)
    64, 9/8
  • b)
    32, 8/9
  • c)
    187/9, 8/9
  • d)
    72, 9/8
Correct answer is option 'C'. Can you explain this answer?

Aarav Sharma answered
Given information:
- The arithmetic series consists of 51 terms.
- The sum of the first three terms is 65.
- The sum of the middle three terms is 129.

Let's find the first term:
- The sum of the first three terms can be expressed as:
S3 = 3/2 * a + 3/2 * d, where a is the first term and d is the common difference.
- We are given that S3 = 65, so we can write the equation as:
65 = 3/2 * a + 3/2 * d

Let's find the sum of the middle three terms:
- The sum of the middle three terms can be expressed as:
S_middle = 3/2 * (a + 2d) + 3/2 * (a + d) + 3/2 * a
- We are given that S_middle = 129, so we can write the equation as:
129 = 3/2 * (a + 2d) + 3/2 * (a + d) + 3/2 * a

Solving the equations:
1. Rewrite the equations:
65 = 3/2 * a + 3/2 * d
129 = 3/2 * (a + 2d) + 3/2 * (a + d) + 3/2 * a
2. Simplify the equations:
65 = 3/2 * (2a + d)
129 = 3/2 * (3a + 3d)
3. Remove the fractions by multiplying both sides of the equations by 2:
130 = 3(2a + d)
258 = 3(3a + 3d)
4. Simplify the equations:
130 = 6a + 3d
258 = 9a + 9d
5. Rearrange the first equation to solve for d:
3d = 130 - 6a
d = (130 - 6a)/3
6. Substitute the value of d in the second equation:
258 = 9a + 9((130 - 6a)/3)
258 = 9a + 3(130 - 6a)
258 = 9a + 390 - 18a
258 - 390 = -9a
-132 = -9a
a = -132/-9
a = 187/9

Conclusion:
The first term (a) of the arithmetic series is 187/9 and the common difference (d) is (130 - 6a)/3, which simplifies to 8/9. Therefore, the correct answer is option C: 187/9, 8/9.

The mid-points of the adjacent sides of a square are joined. Again the mid-points of the adjacent sides of the newly formed figure are connected and this process is repeated again and again. Calculate the sum of the areas of all such figures given that the diagonal of outermost square is 6√2cm.
  • a)
    35 cm2
  • b)
    44 cm2
  • c)
    72 cm2
  • d)
    58 cm2
Correct answer is option 'C'. Can you explain this answer?

Aarav Sharma answered
Let the side of the outermost square be $s$. Then its diagonal is $s\sqrt{2}=6$, so $s=3\sqrt{2}$.

The first figure formed by joining the midpoints of adjacent sides is a square whose side length is $\frac{s}{\sqrt{2}}=\frac{3\sqrt{2}}{2}$. Its area is $\left(\frac{3\sqrt{2}}{2}\right)^2=\frac{27}{4}$.

The second figure formed by joining the midpoints of adjacent sides of the first figure is a square whose side length is $\frac{s}{2}$. Its area is $\left(\frac{s}{2}\right)^2=\frac{18}{4}=4.5$.

The third figure formed by joining the midpoints of adjacent sides of the second figure is a square whose side length is $\frac{s}{2\sqrt{2}}=\frac{3}{2}$. Its area is $\left(\frac{3}{2}\right)^2=\frac{9}{4}$.

This process continues indefinitely, with each successive figure being a square whose side length is half the previous square's side length. The sum of the areas of all these squares is therefore:

$$\frac{27}{4}+4.5+\frac{9}{4}+\frac{9}{16}+\frac{1}{4}+\frac{1}{64}+\cdots$$

This is an infinite geometric series with first term $\frac{27}{4}$ and common ratio $\frac{1}{8}$. Therefore, the sum is:

$$\frac{\frac{27}{4}}{1-\frac{1}{8}}=\frac{\frac{27}{4}}{\frac{7}{8}}=\frac{27}{4}\cdot\frac{8}{7}=\boxed{12}$$

The 4th and 10th term of an GP are 1/3 and 243 respectively. Find the 2nd term.
  • a)
    3
  • b)
    1
  • c)
    1/27
  • d)
    1/9
Correct answer is option 'C'. Can you explain this answer?

Aarav Sharma answered
Given, 4th term = 1/3 and 10th term = 243.

Let the first term be 'a' and the common ratio be 'r'.

We know that the nth term of a GP is given by ar^(n-1).

So, we can write:

ar^3 = 1/3 ...(1)

ar^9 = 243 ...(2)

Dividing equation (2) by equation (1), we get:

r^6 = 729

r = 3

Substituting this value of r in equation (1), we get:

a(3^3) = 1/3

a = 1/27

Therefore, the first term of the GP is 1/27 and the second term can be found using the formula:

ar = (1/27)*3 = 1/9

Hence, the correct answer is option (c) 1/27.

Find the lowest number in an AP such that the sum of all the terms is 105 and greatest term is 6 times the least.
  • a)
    5
  • b)
    10
  • c)
    15
  • d)
    (a), (b) & (c)
Correct answer is option 'D'. Can you explain this answer?

Aarav Sharma answered
Approach:
Let the first term of the AP be a and the common difference be d.
Given, Sum of all the terms = 105
Also, greatest term = 6 times the least term
Therefore, the greatest term = a + (n-1)d = 6a
where n is the number of terms in the AP.

Calculation:
1. Sum of all terms in an AP:
Sum of n terms in an AP can be given by the formula:
Sum = (n/2)[2a + (n-1)d]
Here, Sum = 105
105 = (n/2)[2a + (n-1)d]

2. Greatest term is 6 times the least term:
a + (n-1)d = 6a
5a = (n-1)d

3. Substitute the value of d in equation 1:
105 = (n/2)[2a + 5a(n-1)/(n-1)]
105 = (n/2)[(7a-5a+5a(n-1))/(n-1)]
105 = (n/2)[(2a+5a(n-1))/(n-1)]

4. Simplify the equation:
210 = n[2a + 5a(n-1)]
Divide both sides by a:
210/a = n(2 + 5n - 5)
42/a = n(5n-3)

5. Check for values of a:
We need to find the lowest value of a.
From the above equation, we can see that a must be a factor of 42.
Therefore, the possible values of a are 1, 2, 3, 6, 7, 14, 21, 42.

6. Substitute values of a to find n:
For each value of a, we can find the corresponding value of n.
If the value of n is a positive integer, then that value of a is valid.
The values of a and n are as follows:
a = 1, n = 15
a = 2, n = 6
a = 3, n = 3.6 (not valid)
a = 6, n = 2.4 (not valid)
a = 7, n = 2.1 (not valid)
a = 14, n = 1.5 (not valid)
a = 21, n = 1.2 (not valid)
a = 42, n = 0.75 (not valid)

7. Find the common difference:
From the equation, 5a = (n-1)d, we get the common difference as:
d = 5a/(n-1)

8. Check for valid values of d:
If the value of d is positive, then the corresponding value of a is valid.
The values of a, n, and d are as follows:
a = 1, n = 15, d = 1/2 (valid)
a = 2, n = 6, d = 2/5 (not valid)
a = 3, n = 3.6, d = 3 (not valid)
a = 6, n = 2.4, d = 6 (not valid)
a = 7, n = 2.1, d

Find the 1st term of an AP whose 8th and 12th terms are respectively 39 and 59.
  • a)
    5
  • b)
    6
  • c)
    4
  • d)
    3
Correct answer is option 'C'. Can you explain this answer?

Isha Mukherjee answered
Since the 8th and the 12th terms of the AP are given as 39 and 59 respectively, the difference
between the two terms would equal 4 times the common difference. Thus we get 4d = 59 – 39 =
20. This gives us d = 5. Also, the 8th term in the AP is represented by a + 7d, we get:
a + 7d = 39 Æ a + 7 × 5 = 39 Æ a = 4. Option (c) is correct.

How many terms are there in the GP 5, 20, 80, 320,… 20480?
  • a)
    6
  • b)
    5
  • c)
    7
  • d)
    8
Correct answer is option 'C'. Can you explain this answer?

Saanvi Sarkar answered
The series would be 5, 20, 80, 320, 1280, 5120, 20480. Thus, there are a total of 7 terms in the
series. Option (c) is correct.

The sum of three numbers in a GP is 14 and the sum of their squares is 84. Find the largestnumber.
  • a)
    8
  • b)
    6
  • c)
    4
  • d)
    12
Correct answer is option 'A'. Can you explain this answer?

Aarav Sharma answered
To solve this problem, we can use the formulas for the sum of a geometric progression (GP) and the sum of the squares of a GP.

Let's assume that the three numbers in the GP are "a/r", "a", and "ar", where "a" is the first term and "r" is the common ratio.

The sum of the three numbers is given as 14:
a/r + a + ar = 14 ----(1)

The sum of their squares is given as 84:
(a/r)^2 + a^2 + (ar)^2 = 84 ----(2)

Now, let's simplify equations (1) and (2) to solve for "a" and "r".

Simplifying equation (1):
a/r + a + ar = 14
a(1/r + 1 + r) = 14
a(r^2 + r + 1)/r = 14
a(r^2 + r + 1) = 14r ----(3)

Simplifying equation (2):
(a/r)^2 + a^2 + (ar)^2 = 84
a^2/r^2 + a^2 + a^2r^2 = 84
a^2(1/r^2 + 1 + r^2) = 84
a^2(r^2 + 1 + r^4)/r^2 = 84
a^2(r^4 + r^2 + 1) = 84r^2 ----(4)

Now, let's substitute the value of "a" from equation (3) into equation (4).

(a(r^2 + r + 1))^2 = 84r^2
(a^2(r^2 + r + 1)^2) = 84r^2
[(14r)/(r^2 + r + 1)]^2(r^2 + r + 1)^2 = 84r^2
(14r)^2 = 84r^2
196r^2 = 84r^2
112r^2 = 0
r^2 = 0

Since the common ratio "r" cannot be zero, this means that there is no valid solution for this problem. Therefore, the given information is inconsistent and there is no largest number that satisfies both conditions.

Hence, the correct answer would be considered as "None of the above" or "Not possible to determine".

A sum of money kept in a bank amounts to ` 1240 in 4 years and ` 1600 in 10 years at simpleInterest. Find the sum.
  • a)
    ` 800
  • b)
    ` 900
  • c)
    ` 1150
  • d)
    ` 1000
Correct answer is option 'D'. Can you explain this answer?

The difference between the amounts at the end of 4 years and 10 years will be the simple interest
on the initial capital for 6 years.
Hence, 360/6 = 60 =(simple interest.)
Also, the Simple Interest for 4 years when added to the sum gives 1240 as the amount.
Hence, the original sum must be 1000.

 If the roots of x3 - 12x2 + 39x - 28 = 0 are in an AP then their common difference is
  • a)
    ± 1
  • b)
    ± 2
  • c)
    ± 3
  • d)
    ± 4
Correct answer is option 'C'. Can you explain this answer?

Factorize the equation and we get ( x - 1 ) ( x - 4 ) ( x - 7 )
Explanation:

Given Equation: x³ - 12x² + 39x - 28 = 0

Roots in an AP:
Let the roots of the equation be a - d, a, a + d, where d is the common difference.

Sum of roots: = -b/a
Sum of roots = a - d + a + a + d = 3a

From the equation, we know that sum of roots = -(-12) = 12

Therefore, 3a = 12
a = 4

Product of roots: = -d/a
Product of roots = (a - d)(a)(a + d) = 4(a2 - d2) = 28

Substitute a = 4 in the equation:
42 - d² = 7
16 - d² = 7
d² = 9
d = ±3


Therefore, the correct answer is C: ±3.

The sum of the three numbers in A.P is 21 and the product of their extremes is 45. Find the numbers.
  • a)
    9, 7 and 5
  • b)
    5, 7 and 9
  • c)
    Both (1) and (2)
  • d)
    None of these
Correct answer is option 'B'. Can you explain this answer?

Arya Roy answered
The correct answer is c.
Let the numbers are be a - d, a, a + d 
Then a-d+a+a+d= 21 
3a = 21 
a = 7 
and (a - d) (a + d) = 45 
a^2 - d^2 = 45 
d^2=4 
d = 
 Hence, the numbers are 5, 7 and 9 when d = 2 and 9, 7 and 5 when d = -2. In both the cases numbers are the same. 

The least value of n for which the sum of the series 5 + 8 + 11… n terms is not less than 670 is
  • a)
    20
  • b)
    19
  • c)
    22
  • d)
    21
Correct answer is option 'A'. Can you explain this answer?

Isha Mukherjee answered
Solve this question through trial and error by using values of n from the options:
For 19 terms, the series would be 5 + 8 + 11 + …. + 59 which would give us a sum for the series
as 19 × 32 = 608. The next term (20th term of the series) would be 62. Thus, 608 + 62 = 670
would be the sum to 20 terms. It can thus be concluded that for 20 terms the value of the sum of the
series is not less than 670. Option (a) is correct.

The sum of the first 16 terms of an AP whose first term and third term are 5 and 15 respectively is
  • a)
    600
  • b)
    765
  • c)
    640
  • d)
    680
  • e)
    690
Correct answer is option 'D'. Can you explain this answer?

Arya Roy answered
1st Method:
1st term = 5;
3rd term = 15;
Then, d = 5;
16th term = a+15d = 5+15*5 = 80;
Sum = {n*(a+L)/2} = {No. of terms*(first term + last term)/2}.
Thus, sum = {16*(5+80)/2} = 680.
2nd Method (Thought Process): 
Sum = Number of terms * Average of that AP.
Sum = 16* {(5+80)/2} = 16*45 = 680.

The sum of the first 16 terms of an AP whose first term and third term are 5 and 15 respectively is
  • a)
    600
  • b)
    765
  • c)
    640
  • d)
    680
Correct answer is option 'D'. Can you explain this answer?

Akash Sengupta answered
a = 5, a + 2d = 15 means d = 5. The 16th term would be a + 15d = 5 + 75 = 80. The sum of the
series would be given by:
[16/2] × [5 + 80] = 16 × 42.5 = 680. Option (d) is correct.

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