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
Find four numbers in A.P. whose sum is 20 and the sum of whose squares is 120
Three numbers are in A.P. If the sum of these numbers is 27 and the product 648, find the numbers.
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 all natural numbers between 200 and 400 which are divisible by 7.
The sum of first n terms of an A.P is 5n2 + 3n. If its mth term is 168, find the value of m. Also, find the 20th term of this A.P.
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.
Q.15
Find the common difference of an A.P. whose first term is 5 and the sum of first four terms is half the sum of next four terms.
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]
Find the sum of first 20 terms of an A.P. whose nth term is given as an = 5 – 2n.
