Advertisements
Advertisements
प्रश्न
Find the sum of first 20 terms of an A.P. whose nth term is given as an = 5 – 2n.
उत्तर
We have, an = 5 – 2n
∴ a1 = 5 – 2 = 3
and a2 = 5 – 4 = 1
∴ d(common difference) = 1 – 3 = –2
Now, S20 = `20/2[2 xx 3 + (20 - 1)(-2)]` ......`[∵ S_n = n/2[2a + (n - 1)d]]`
= 10[6 – 19 × 2]
= 10[6 – 38]
= 10 × (–32)
= –320
APPEARS IN
संबंधित प्रश्न
Which term of the progression 20, 19`1/4`,18`1/2`,17`3/4`, ... is the first negative term?
The 9th term of an AP is -32 and the sum of its 11th and 13th terms is -94. Find the common difference of the AP.
What is the sum of first 10 terms of the A. P. 15,10,5,........?
If the sum of first n terms of an A.P. is \[\frac{1}{2}\] (3n2 + 7n), then find its nth term. Hence write its 20th term.
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.
The sum of n terms of an A.P. is 3n2 + 5n, then 164 is its
A merchant borrows ₹ 1000 and agrees to repay its interest ₹ 140 with principal in 12 monthly instalments. Each instalment being less than the preceding one by ₹ 10. Find the amount of the first instalment
Find the sum of those integers from 1 to 500 which are multiples of 2 or 5.
[Hint (iii) : These numbers will be : multiples of 2 + multiples of 5 – multiples of 2 as well as of 5]
The nth term of an Arithmetic Progression (A.P.) is given by the relation Tn = 6(7 – n)..
Find:
- its first term and common difference
- sum of its first 25 terms
