Advertisements
Advertisements
Question
Choose the correct alternative answer for the following question .
In an A.P. first two terms are –3, 4 then 21st term is ...
Options
-143
143
137
17
Advertisements
Solution
It is given that,
a = –3
a2 = 4
We know that,
\[a_2 = a + \left( 2 - 1 \right)d\]
\[ \Rightarrow 4 = - 3 + d\]
\[ \Rightarrow d = 7\]
Now,
\[a_{21} = a + \left( 21 - 1 \right)d\]
\[ = - 3 + 20\left( 7 \right)\]
\[ = - 3 + 140\]
\[ = 137\]
APPEARS IN
RELATED QUESTIONS
Find four numbers in A.P. whose sum is 20 and the sum of whose squares is 120
If (m + 1)th term of an A.P is twice the (n + 1)th term, prove that (3m + 1)th term is twice the (m + n + 1)th term.
Find the sum of the following arithmetic progressions:
a + b, a − b, a − 3b, ... to 22 terms
Find the sum of the first 15 terms of each of the following sequences having the nth term as
bn = 5 + 2n
Find the sum of the first 15 terms of each of the following sequences having the nth term as
yn = 9 − 5n
How many terms of the A.P. : 24, 21, 18, ................ must be taken so that their sum is 78?
Find the middle term of the AP 6, 13, 20, …., 216.
Which term of the A.P. `20, 19 1/4, 18 1/2, 17 3/4,` ..... is the first negative term?
The sum of three consecutive terms of an AP is 21 and the sum of the squares of these terms is 165. Find these terms
Find the sum of all multiples of 9 lying between 300 and 700.
Write an A.P. whose first term is a and common difference is d in the following.
a = 6, d = –3
The first term of an A. P. is 5 and the common difference is 4. Complete the following activity and find the sum of the first 12 terms of the A. P.
a = 5, d = 4, s12 = ?
`s_n = n/2 [ square ]`
`s_12 = 12/2 [10 +square]`
`= 6 × square `
` =square`
In a school, students decided to plant trees in and around the school to reduce air pollution. It was decided that the number of trees, that each section of each class will plant, will be double of the class in which they are studying. If there are 1 to 12 classes in the school and each class has two sections, find how many trees were planted by the students.
Write the common difference of an A.P. whose nth term is an = 3n + 7.
Write the sum of first n even natural numbers.
Q.2
Q.14
Show that a1, a2, a3, … form an A.P. where an is defined as an = 3 + 4n. Also find the sum of first 15 terms.
The sum of first n terms of the series a, 3a, 5a, …….. is ______.
k + 2, 2k + 7 and 4k + 12 are the first three terms of an A.P. The first term of this A.P. is ______.
