Advertisements
Advertisements
प्रश्न
In an A.P., the sum of its first n terms is 6n – n². Find is 25th term.
Advertisements
उत्तर
Sn = 6n – n²
T25 = ?
S(n–1) = 6(n – 1) – (n – 1)2
= 6n – 6 – (n2 – 2n + 1)
= 6n – 6 – n2 + 2n –1
= 8n – n2 – 7
an = Sn – Sn – 1
= 6n – n2 – 8n + n2 + 7
= –2n + 7
a25 = –2(25) + 7
= –50 + 7
= –43.
APPEARS IN
संबंधित प्रश्न
In an AP: Given a = 5, d = 3, an = 50, find n and Sn.
In a potato race, a bucket is placed at the starting point, which is 5 m from the first potato and other potatoes are placed 3 m apart in a straight line. There are ten potatoes in the line.
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)]
Find how many integers between 200 and 500 are divisible by 8.
Find the sum of n terms of an A.P. whose nth terms is given by an = 5 − 6n.
Find the sum of all integers between 84 and 719, which are multiples of 5.
Find the sum of the first 15 terms of each of the following sequences having the nth term as
yn = 9 − 5n
Find the sum of all natural numbers between 250 and 1000 which are divisible by 9.
The first three terms of an AP are respectively (3y – 1), (3y + 5) and (5y + 1), find the value of y .
If (2p – 1), 7, 3p are in AP, find the value of p.
Q.13
