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Q17:
Find the 20th term from the last term of the AP : 3, 8, 13, . . ., 253.
Solution :
Given: An Arithmetic Progression (AP) sequence: $3, 8, 13, \dots, 253$.
To Find: The $20^{th}$ term from the end (last term) of the given AP.
Step 1: Identify the parameters of the given AP.
The sequence is $3, 8, 13, \dots, 253$.
First term ($a$) = $3$.
Common difference ($d$) = $a_2 - a_1 = 8 - 3 = 5$.
Last term ($l$) = $253$.
Step 2: Formulate the strategy to find the $n^{th}$ term from the end.
To find the $n^{th}$ term from the end of an AP, we can reverse the sequence. The new AP will have the last term of the original AP as its first term, and the common difference will be the negative of the original common difference ($-d$).
Let the reversed AP be: $253, 248, 243, \dots, 3$.
For this reversed AP:
New first term ($a'$) = $253$.
New common difference ($d'$) = $-5$.
We need to find the $20^{th}$ term ($n = 20$).
Step 3: Apply the formula for the $n^{th}$ term of an AP.
The formula for the $n^{th}$ term of an AP is given by:
$a_n = a' + (n - 1)d'$
[Where $a_n$ is the $n^{th}$ term, $a'$ is the first term, $n$ is the position, and $d'$ is the common difference.]
Step 4: Substitute the values into the formula.
$a_{20} = 253 + (20 - 1)(-5)$
$a_{20} = 253 + (19)(-5)$
[Performing the multiplication: $19 \times -5 = -95$]
$a_{20} = 253 - 95$
Step 5: Perform the final subtraction.
$a_{20} = 158$
Final Answer: The 20th term from the last term of the AP is 158.
More Questions from Class 10 Mathematics Arithmetic Progression EXERCISE 5.2
- Q1(i): Fill in the blanks in the following table, given that $a$ is the first term, $d$ the common difference and $a_n$ the $n$th term of the AP: (i) $a=7, d=3, n=8, a_n=...$
- Q1(ii): Fill in the blanks in the following table, given that $a$ is the first term, $d$ the common difference and $a_n$ the $n$th term of the AP: (ii) $a=–18, d=..., n=10, a_n=0$
- Q1(iii): Fill in the blanks in the following table, given that $a$ is the first term, $d$ the common difference and $a_n$ the $n$th term of the AP: (iii) $a=..., d=–3, n=18, a_n=–5$
- Q1(iv): Fill in the blanks in the following table, given that $a$ is the first term, $d$ the common difference and $a_n$ the $n$th term of the AP: (iv) $a=–18.9, d=2.5, n=..., a_n=3.6$
- Q1(v): Fill in the blanks in the following table, given that $a$ is the first term, $d$ the common difference and $a_n$ the $n$th term of the AP: (v) $a=3.5, d=0, n=105, a_n=...$
- Q10: The 17th term of an AP exceeds its 10th term by 7. Find the common difference.
- Q11: Which term of the AP : 3, 15, 27, 39, . . . will be 132 more than its 54th term?
- Q12: Two APs have the same common difference. The difference between their 100th terms is 100, what is the difference between their 1000th terms?
- Q13: How many three-digit numbers are divisible by 7?
- Q14: How many multiples of 4 lie between 10 and 250?
- Q15: For what value of $n$, are the $n$th terms of two APs: 63, 65, 67, . . . and 3, 10, 17, . . . equal?
- Q16: Determine the AP whose third term is 16 and the 7th term exceeds the 5th term by 12.
- Q18: The sum of the 4th and 8th terms of an AP is 24 and the sum of the 6th and 10th terms is 44. Find the first three terms of the AP.
- Q19: Subba Rao started work in 1995 at an annual salary of ₹ 5000 and received an increment of ₹ 200 each year. In which year did his income reach ₹ 7000?
- Q2(i): Choose the correct choice in the following and justify : (i) 30th term of the AP: 10, 7, 4, . . . , is
- Q2(ii): Choose the correct choice in the following and justify : (ii) 11th term of the AP: – 3, $-\frac{1}{2}$, 2, . . ., is
- Q20: Ramkali saved ₹ 5 in the first week of a year and then increased her weekly savings by ₹ 1.75. If in the $n$th week, her weekly savings become ₹ 20.75, find $n$.
- Q3(i): In the following APs, find the missing terms in the boxes : (i) 2, ☐, 26
- Q3(ii): In the following APs, find the missing terms in the boxes : (ii) ☐, 13, ☐, 3
- Q3(iii): In the following APs, find the missing terms in the boxes : (iii) 5, ☐, ☐, $9\frac{1}{2}$
- Q3(iv): In the following APs, find the missing terms in the boxes : (iv) – 4, ☐, ☐, ☐, ☐, 6
- Q3(v): In the following APs, find the missing terms in the boxes : (v) ☐, 38, ☐, ☐, ☐, – 22
- Q4: Which term of the AP : 3, 8, 13, 18, . . . ,is 78?
- Q5(i): Find the number of terms in each of the following APs : (i) 7, 13, 19, . . . , 205
- Q5(ii): Find the number of terms in each of the following APs : (ii) 18, $15\frac{1}{2}$, 13, . . . , – 47
- Q6: Check whether – 150 is a term of the AP : 11, 8, 5, 2 . . .
- Q7: Find the 31st term of an AP whose 11th term is 38 and the 16th term is 73.
- Q8: An AP consists of 50 terms of which 3rd term is 12 and the last term is 106. Find the 29th term.
- Q9: If the 3rd and the 9th terms of an AP are 4 and – 8 respectively, which term of this AP is zero?
CBSE Solutions for Class 10 Mathematics Arithmetic Progression
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