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
If the mth term of an A.P. is 1/n and the nth term is 1/m, show that the sum of mn terms is (mn + 1)
Find the sum of the following APs.
`1/15, 1/12, 1/10`, ......, to 11 terms.
In an A.P., the sum of first n terms is `(3n^2)/2 + 13/2 n`. Find its 25th term.
The sum of first three terms of an AP is 48. If the product of first and second terms exceeds 4 times the third term by 12. Find the AP.
Find the sum of the first n natural numbers.
Choose the correct alternative answer for the following question .
15, 10, 5,... In this A.P sum of first 10 terms is...
The common difference of an A.P., the sum of whose n terms is Sn, is
Find the sum of first 10 terms of the A.P.
4 + 6 + 8 + .............
Find the sum of all 11 terms of an A.P. whose 6th term is 30.
In a ‘Mahila Bachat Gat’, Kavita invested from the first day of month ₹ 20 on first day, ₹ 40 on second day and ₹ 60 on third day. If she saves like this, then what would be her total savings in the month of February 2020?
