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
Find the sum of the first n natural numbers.
Find the first term and common difference for the A.P.
0.6, 0.9, 1.2,1.5,...
The sum of first n terms of an A.P. is 5n − n2. Find the nth term of this A.P.
Write the nth term of the \[A . P . \frac{1}{m}, \frac{1 + m}{m}, \frac{1 + 2m}{m}, . . . .\]
If the sum of three consecutive terms of an increasing A.P. is 51 and the product of the first and third of these terms is 273, then the third term is
Find whether 55 is a term of the A.P. 7, 10, 13,... or not. If yes, find which term is it.
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.
The sum of the first 15 multiples of 8 is ______.
If the sum of first n terms of an AP is An + Bn² where A and B are constants. The common difference of AP will be ______.
