NCERT Solutions for Class 10 Maths Chapter 5 Exercise 5.3 focus on finding the sum of an arithmetic progression. This exercise introduces formulas that help you calculate the total of a given number of terms without listing the entire sequence, making it an important part of the CBSE 10th Maths syllabus.
These NCERT Solutions are explained step by step so you can understand how values like the first term, common difference, and number of terms are used together. Practising these questions will help you improve calculation accuracy and build confidence in solving sequence-based problems.
(i) 2, 7, 12,…, to 10 terms. (ii) − 37, − 33, − 29 ,…, to 12 terms (iii) 0.6, 1.7, 2.8 ,…….., to 100 terms (iv) 1/15, 1/12, 1/10, …… , to 11 terms
Solutions:
(i) Given, 2, 7, 12 ,…, to 10 terms
For this A.P., First term, a = 2 And common difference,
d = a 2 − a 1 = 7−2 = 5
n = 10
We know that the formula for sum of nth term in AP series is, S n = n/2 [2a +(n-1)d]
S 10 = 10/2 [2(2)+(10 -1)×5]
= 5[4+(9)×(5)]
= 5 × 49 = 245
(ii) Given, −37, −33, −29 ,…, to 12 terms
For this A.P., First term, a = −37 And common difference,
d = a 2 − a 1 d
= (−33)−(−37)
= − 33 + 37 = 4
n = 12
We know that the formula for sum of nth term in AP series is,
S n = n/2 [2a+(n-1)d]
S 12 = 12/2 [2(-37)+(12-1)×4]
= 6[-74+11×4]
= 6[-74+44]
= 6(-30)
= -180
iii) Given, 0.6, 1.7, 2.8 ,…, to 100 terms
For this A.P., First term, a = 0.6 Common difference,
d = a 2 − a 1
= 1.7 − 0.6
= 1.1 n = 100
We know that the formula for sum of nth term in AP series is,
S n = n/2[2a +(n-1)d]
S 12 = 50/2 [1.2+(99)×1.1]
= 50[1.2+108.9]
= 50[110.1]
= 5505
(iv) Given, 1/15, 1/12, 1/10, …… , to 11 terms
For this A.P., First term, a = 1/5 Common difference,
d = a 2 –a 1
= (1/12)-(1/5) = 1/60
And number of terms n = 11 We know that the formula for sum of nth term in AP series is, S n = n /2 [2a + ( n – 1) d ]
= 11/2(2/15 + 10/60)
= 11/2 (9/30)
= 33/20
(ii) 34 + 32 + 30 + ……….. + 10
(iii) − 5 + (− 8) + (− 11) + ………… + (− 230)
Solutions:
(i)
First term, a = 7 n th term, a n = 84
Let 84 be the n th term of this A.P., then as per the n th term formula,
a n = a(n-1)d 84
= 7+(n – 1)×7/2 77
= (n-1)×7/2 22
= n−1
n =23
We know that the sum of n term is; S n = n/2 (a + l) , l = 84 S n = 23/2 (7+84) S n = (23×91/2) = 2093/2
(ii) Given, 34 + 32 + 30 + ……….. + 10
For this A.P., First term, a = 34
Common difference,
d = a 2 −a 1
= 32−34 = −2
n th term,
a n = 10
Let 10 be the n th term of this A.P., therefore, a n = a +(n−1)d 10
= 34+(n−1)(−2) −24
= (n −1)(−2) 12
= n −1
n = 13
We know that the sum of n terms is; S n = n/2 (a +l) , l = 10
= 13/2 (34 + 10)
= (13×44/2)
= 13 × 22
= 286
(iii) Given, (−5) + (−8) + (−11) + ………… + (−230)
For this A.P., First term, a = −5 nth term, a n = −230
Common difference,
d = a 2 −a 1 = (−8)−(−5)
⇒d = − 8+5 = −3
Let −230 be the n th term of this A.P., and by the nth term formula
we know, a n = a +( n −1) d −230
= − 5+( n −1)(−3) −225
= ( n −1)(−3) ( n −1) = 75
n = 76
And the sum of n term, S n = n /2 ( a + l )
= 76/2 [(-5) + (-230)]
= 38(-235)
= -8930
(i) Given a = 5, d = 3, a n = 50, find n and S n .
(ii) Given a = 7, a 13 = 35, find d and S 13 .
(iii) Given a 12 = 37, d = 3, find a and S 12 .
(iv) Given a 3 = 15, S 10 = 125, find d and a 10 .
(v) Given d = 5, S 9 = 75, find a and a 9 .
(vi) Given a = 2, d = 8, S n = 90, find n and a n .
(vii) Given a = 8, a n = 62, S n = 210, find n and d .
(viii) Given a n = 4, d = 2, S n = − 14, find n and a .
(ix) Given a = 3, n = 8, S = 192, find d .
(x) Given l = 28, S = 144 and there are total 9 terms. Find a .
Solutions:
(i) Given that, a = 5, d = 3, a n = 50
As we know, from the formula of the nth term in an AP, a n = a +( n −1) d ,
Therefore, putting the given values, we get,
⇒ 50 = 5+( n -1)×3
⇒ 3( n -1) = 45
⇒ n -1 = 15
⇒ n = 16
Now, sum of n terms, S n = n /2 ( a + a n )
S n = 16/2 (5 + 50) = 440
(ii) Given that, a = 7, a 13 = 35
As we know, from the formula of the nth term in an AP, a n = a +( n −1) d ,
Therefore, putting the given values, we get,
⇒ 35 = 7+(13-1) d
⇒ 12 d = 28
⇒ d = 28/12
= 2.33
Now, S n = n /2 ( a + a n )
S 13 = 13/2 (7+35) = 273
(iii) Given that, a 12 = 37, d = 3
As we know, from the formula of the n th term in an AP, a n = a +( n −1) d ,
Therefore, putting the given values, we get,
⇒ a 12 = a +(12−1)3
⇒ 37 = a +33
⇒ a = 4
Now, sum of nth term, S n = n /2 ( a + a n )
S n = 12 /2 (4+37) = 246
(iv) Given that, a 3 = 15, S 10 = 125
As we know, from the formula of the nth term in an AP, a n = a +( n −1) d.
Therefore, putting the given values, we get, a 3 = a +(3−1) d 15 = a +2 d ………………………….. (i)
Sum of the nth term, S n = n /2 [2 a +( n -1) d ]
S 10 = 10/2 [2 a +(10-1) d ]
125 = 5(2 a +9 d ) 25 = 2 a +9 d ……………………….. (ii)
On multiplying equation (i) by (ii) , we will get; 30 = 2 a +4 d ………………………………. (iii)
By subtracting equation (iii) from (ii) , we get, −5 = 5 d
d = −1
From equation (i) , 15 = a +2(−1) 15
= a −2 a = 17 =
First term a 10 = a +(10−1) d
a 10 = 17+(9)(−1) a 10
= 17−9
= 8
(v) Given that, d = 5, S 9 = 75
As, sum of n terms in AP is, S n = n /2 [2 a +( n -1) d ]
Therefore, the sum of first nine terms are; S 9 = 9/2 [2 a +(9-1) 5 ] 25
= 3( a +20) 25
= 3 a +60 3 a
= 25−60 a
= -35/3
As we know, the n th term can be written as; a n = a +( n −1) d a 9
= a +(9−1)(5)
= -35/3+8(5)
= -35/3+40
= (35+120/3)
= 85/3
(vi) Given that, a = 2, d = 8, S n = 90
As, sum of n terms in an AP is,
S n = n /2 [2 a +( n -1) d ]
90 = n /2 [2 a +( n -1) d ]
⇒ 180 = n (4+8 n -8) = n (8 n -4) = 8 n 2 -4 n
⇒ 8 n 2 -4 n – 180 = 0
⇒ 2 n 2 – n -45 = 0
⇒ 2 n 2 -10 n +9 n -45 = 0
⇒ 2 n ( n -5)+9( n -5) = 0
⇒ ( n -5)(2 n +9) = 0
So, n = 5 (as n only is a positive integer) ∴ a 5 = 8+5×4 = 34
(vii) Given that, a = 8, a n = 62, S n = 210
As, the sum of n terms in an AP is, S n = n /2 ( a + a n ) 210 = n /2 (8 +62) ⇒ 35 n = 210 ⇒ n = 210/35 = 6
Now, 62 = 8+5 d
⇒ 5 d = 62-8 = 54
⇒ d = 54/5 = 10.8
(viii) Given that, n th term, a n = 4, common difference, d = 2, sum of n terms, S n = −14.
As we know, from the formula of the n th term in an AP, a n = a +( n −1) d ,
Therefore, putting the given values, we get, 4 = a +( n −1)2 4
= a +2 n −2 a +2 n
= 6 a = 6 − 2 n …………………………………………. (i)
As we know, the sum of n terms is; S n = n /2 ( a + a n ) -14 = n /2 ( a + 4 ) −28 = n ( a +4) −28 = n (6 −2 n +4)
{From equation (i) } −28 = n (− 2 n +10) −28 = − 2 n 2 +10 n 2 n 2 −10 n − 28 = 0 n 2 −5 n −14 = 0 n 2 −7 n+ 2 n −14 = 0 n ( n −7)+2( n −7) = 0 ( n −7)( n +2) = 0 Either n − 7 = 0 or n + 2 = 0 n = 7 or n = −2
However, n can neither be negative nor fractional.
Therefore, n = 7 From equation (i) , we get a = 6−2 n a = 6−2(7) = 6−14 = −8
(ix) Given that, first term, a = 3, Number of terms, n = 8
And sum of n terms, S = 192 As we know,
S n = n /2 [2 a +( n -1) d ]
192 = 8/2 [2×3+(8 -1) d ]
192 = 4[6 +7 d ] 48 = 6+7 d
42 = 7 d d = 6 (x)
Given that, l = 28, S = 144 and there are total of 9 terms.
Sum of n terms formula, S n = n /2 ( a + l )
144 = 9/2( a +28) (16)×(2)
= a +28 32
= a +28
a = 4
Solutions:
Let there be n terms of the AP. 9, 17, 25 … For this A.P.,
First term, a = 9 Common difference,
d = a 2 − a 1 = 17−9 = 8
As, the sum of n terms, is;
S n = n /2 [2 a +( n -1) d ]
636 = n /2 [2× a +(8-1)×8]
636 = n /2 [18+( n -1)×8]
636 = n [9 +4 n −4]
636 = n (4 n +5) 4 n 2 +5 n
−636 = 0 4 n 2 +53 n −48 n
−636 = 0 n (4 n + 53)−12 (4 n + 53) = 0 (4 n +53)( n −12) = 0
Either 4 n +53 = 0 or n −12 = 0 n = (-53/4) or n = 12 n cannot be negative or fraction, therefore, n = 12 only.
Solution:
Given that, first term, a = 5 last term, l = 45 Sum of the AP, S n = 400
As we know, the sum of AP formula is; S n = n /2 ( a + l )
400 = n /2(5+45)
400 = n /2(50)
Number of terms, n =16
As we know, the last term of AP series can be written as; l = a+ ( n −1) d
45 = 5 +(16 −1) d
40 = 15 d
Common difference, d = 40/15 = 8/3
Solution:
Given that, First term, a = 17 Last term, l = 350 Common difference, d = 9 Let there be n terms in the A.P., thus the formula for last term can be written as;
l = a+ ( n −1) d
350 = 17+( n −1)9
333 = ( n −1)9 ( n −1) = 37
n = 38 S n = n /2 ( a + l )
S 38 = 38/2 (17+350) = 19×367 = 6973
Thus, this A.P. contains 38 terms and the sum of the terms of this A.P. is 6973.
Solution:
Given, Common difference, d = 7 22 nd term, a 22 = 149
Sum of first 22 term, S 22 = ?
By the formula of nth term, a n = a +( n −1) d
a 22 = a +(22−1) d
149 = a +21×7
149 = a +147 a = 2 =
First term Sum of n terms, S n = n /2( a + a n )
S 22 = 22/2 (2+149) = 11×151 = 1661
Solution:
Given that, Second term, a 2 = 14 Third term, a 3 = 18
Common difference,
d = a 3 − a 2 = 18−14 = 4
a 2 = a + d
14 = a +4
a = 10 = First term Sum of n terms;
S n = n /2 [2 a + ( n – 1) d ]
S 51 = 51/2 [2×10 (51-1) 4]
= 51/2 [20+(50)×4]
= 51 × 220/2
= 51 × 110
= 5610
Solution:
Given that, S 7 = 49 S 17 = 289
We know, sum of n terms; S n = n /2 [2 a + ( n – 1) d ]
Therefore, S 7 = 7 /2 [2 a +( n -1) d ]
S 7 = 7/2 [2 a + (7 -1) d ]
49 = 7/2 [2 a + 6 d ]
7 = ( a +3 d ) a + 3 d = 7 …………………………………. (i)
In the same way, S 17 = 17/2 [2 a +(17-1) d ]
289 = 17/2 (2 a +16 d )
17 = ( a +8 d ) a +8 d = 17 ………………………………. (ii)
Subtracting equation (i) from equation (ii) , 5 d = 10 d = 2
From equation (i) , we can write it as; a +3(2) = 7 a+ 6 = 7 a = 1
Hence, S n = n /2[2 a +( n -1) d ]
= n /2[2(1)+( n – 1)×2]
= n /2(2+2 n -2)
= n /2(2 n )
= n 2
(i) a n = 3+4 n
(ii) a n = 9−5 n.
Also, find the sum of the first 15 terms in each case.
Solutions:
(i) a n = 3+4 n
a 1 = 3+4(1) = 7
a 2 = 3+4(2) = 3+8 = 11
a 3 = 3+4(3) = 3+12 = 15
a 4 = 3+4(4) = 3+16 = 19
We can see here, the common difference between the terms are; a 2 − a 1 = 11−7 = 4
a 3 − a 2 = 15−11 = 4
a 4 − a 3 = 19−15 = 4
Hence, a k + 1 − a k is the same value every time.
Therefore, this is an AP with common difference as 4 and first term as 7. Now, we know, the sum of nth term is;
S n = n /2[2 a +( n -1) d ]
S 15 = 15/2[2(7)+(15-1)×4]
= 15/2[(14)+56]
= 15/2(70)
= 15×35
= 525
(ii) a n = 9−5 n a 1 = 9−5×1 = 9−5 = 4 a 2 = 9−5×2 = 9−10 = −1 a 3 = 9−5×3 = 9−15 = −6 a 4 = 9−5×4 = 9−20 = −11
We can see here, the common difference between the terms are;
a 2 − a 1 = −1−4 = −5
a 3 − a 2 = −6−(−1) = −5
a 4 − a 3 = −11−(−6) = −5
Hence, a k + 1 − a k is the same every time.
Therefore, this is an A.P. with common difference as −5 and a first term as 4.Now, we know, the sum of nth term is; S n = n /2 [2 a +( n -1) d ]
S 15 = 15/2[2(4) +(15 -1)(-5)] =
15/2[8 +14(-5)]
= 15/2(8-70)
= 15/2(-62)
= 15(-31)
= -465
Solution:
Given that, S n = 4 n − n 2 First term, a = S 1 = 4(1) − (1) 2 = 4−1 = 3
Sum of first two terms = S 2 = 4(2)−(2) 2 = 8−4 = 4
Second term, a 2 = S 2 − S 1 = 4−3 = 1
Common difference, d = a 2 − a = 1−3 = −2
N th term, a n = a +( n −1) d = 3+( n −1)(−2) = 3−2 n +2 = 5−2 n
Therefore, a 3 = 5−2(3)
= 5-6 = −1
a 10 = 5−2(10) = 5−20 = −15
Hence, the sum of first two terms is 4. The second term is 1. The 3rd, the 10th, and the n th terms are −1, −15, and 5 − 2 n respectively.
Solution:
The positive integers that are divisible by 6 are 6, 12, 18, 24 ….
We can see here that this series forms an A.P. whose first term is 6 and the common difference is 6. a = 6 d = 6 S 40 = ?
By the formula of sum of n terms, we know, S n = n /2 [2 a +( n – 1) d ]
Therefore, putting n = 40, we get,
S 40 = 40/2 [2(6)+(40-1)6]
= 20[12+(39)(6)]
= 20(12+234)
= 20×246
= 4920
Solution:
The multiples of 8 are 8, 16, 24, 32… The series is in the form of AP, having first term as 8 and common difference as 8.
Therefore, a = 8 d = 8 S 15 = ? By the formula of sum of nth term, we know,
S n = n /2 [2 a +( n -1) d ]
S 15 = 15/2 [2(8) + (15-1)8]
= 15/2[16 +(14)(8)]
= 15/2[16 +112]
= 15(128)/2
= 15 × 64
= 960
Solution:
The odd numbers between 0 and 50 are 1, 3, 5, 7, 9 … 49.
Therefore, we can see that these odd numbers are in the form of A.P.
Hence, First term, a = 1 Common difference, d = 2 Last term, l = 49 By the formula of last term,
we know, l = a +( n −1) d
49 = 1+( n −1)2
48 = 2( n − 1)
− 1 = 24 n = 25 = Number of terms. By the formula of sum of nth term,
we know, S n = n /2( a + l )
S 25 = 25/2 (1+49)
= 25(50)/2
=(25)(25)
= 625
Solution:
We can see that the given penalties are in the form of A.P., having the first term as 200 and a common difference of 50.
Therefore, a = 200 and d = 50. The penalty that has to be paid if the contractor has delayed the work by 30 days = S 30
By the formula of sum of nth term, we know, S n = n /2[2 a +( n -1) d ]
Therefore, S 30 = 30/2[2(200)+(30 – 1)50]
= 15[400+1450]
= 15(1850)
= 27750.
Therefore, the contractor has to pay Rs 27750 as a penalty.
Solution:
Let the cost of 1 st prize be Rs. P..
Cost of 2 nd prize = Rs. P − 20 And
cost of 3 rd prize = Rs. P − 40
we can see that the cost of these prizes are in the form of A.P., having a common difference as −20 and first term as P .
Thus, a = P and d = −20 Given that, S 7 = 700 By the formula of sum of nth term,
we know, S n = n /2 [2 a + ( n – 1) d ] 7/2 [2 a + (7 – 1) d ] = 700
a + 3(−20) = 100 a −60 = 100 a = 160
Therefore, the value of each of the prizes was Rs 160, Rs 140, Rs 120, Rs 100, Rs 80, Rs 60, and Rs 40.
Solution:
It can be observed that the number of trees planted by the students is in an AP. 1, 2, 3, 4, 5………………..12
First term, a = 1
Common difference,
d = 2−1 = 1
S n = n /2 [2 a +( n -1) d ]
S 12 = 12/2 [2(1)+(12-1)(1)] = 6(2+11) = 6(13) = 78
Therefore, the number of trees planted by 1 section of the classes = 78.
The number of trees planted by 3 sections of the classes = 3×78 = 234.
Therefore, 234 trees will be planted by the students.
Solution:
We know, Perimeter of a semi-circle = π r Therefore, P 1 = π(0.5) = π/2 cm P 2 = π(1) = π cm P 3 = π(1.5) = 3π/2 cm
Where, P 1, P 2 , P 3 are the lengths of the semi-circles.
Hence we got a series here, as, π/2, π, 3π/2, 2π, …. P 1 = π/2 cm P 2 = π cm
Common difference, d = P 2 – P 1 = π – π/2 = π/2
First term = P 1 = a = π/2 cm
By the sum of n term formula, we know, S n = n /2 [2 a + ( n – 1) d ]
Therefore, the sum of the length of 13 consecutive circles is: S 13 = 13 /2 [2(π/2) + (13 – 1)π/2]
= 13 /2 [π + 6π]
=13 /2 (7π)
= 13 /2 × 7 × 22 /7
= 143 cm
Solution:
We can see that the numbers of logs in rows are in the form of an A.P. 20, 19, 18… For the given A.P., First term, a = 20 and common difference, d = a 2 − a 1 = 19−20 = −1.
Let a total of 200 logs be placed in n rows. Thus, S n = 200 By the sum of nth term formula, S n = n /2 [2 a +( n -1) d ]
S 12 = 12/2 [2(20)+( n -1)(-1)]
400 = n (40− n +1)
400 = n (41- n )
400 = 41 n − n 2 n 2 −41 n + 400 = 0 n 2 −16 n −25 n +400 = 0 n ( n −16)−25( n −16) = 0 ( n −16)( n −25) = 0 Either ( n −16) = 0 or n −25 = 0 n = 16 or n = 25
By the nth term formula, a n = a +( n −1) d
a 16 = 20+(16−1)(−1)
a 16 = 20−15 a 16 = 5
Similarly, the 25 th term could be written as; a 25 = 20+(25−1)(−1)
a 25 = 20−24 = − 4
It can be seen, the number of logs in the 16 th row is 5 as the numbers cannot be negative.
Therefore, 200 logs can be placed in 16 rows, and the number of logs in the 16th row is 5.
A competitor starts from the bucket, picks up the nearest potato, runs back with it, drops it in the bucket, runs back to pick up the next potato, runs to the bucket to drop it in, and she continues in the same way until all the potatoes are in the bucket. What is the total distance the competitor has to run? [Hint: to pick up the first potato and the second potato, the total distance (in metres) run by a competitor is 2×5+2×(5+3)]
Solution:
The distances of the potatoes from the bucket are 5, 8, 11, 14…, which is in the form of AP.
Given the distance run by the competitor for collecting these potatoes are two times of the distance at which the potatoes have been kept.
Therefore, the distance to be run w.r.t the distances of potatoes could be written as; 10, 16, 22, 28, 34,……….
Hence, the first term, a = 10 and d = 16−10 = 6 S 10 =?
By the formula of sum of n terms, we know, S 10 = 10/2 [2(10)+(10 -1)(6)] = 5[20+54] = 5(74) = 370.
Therefore, the competitor will run a total distance of 370 m.
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