Question

-0. In an A.P. the sum of first n terms is 3
+ 131. Find the 25th term.
2
2​

Answer 1

Step-by-step explanation:

n term sum = 3n²/2 + 13n/2

as we know that nth term = (Sum of nth term ) -( sum of (n-1)th term)

method 1

so 25th term = (sum of 25 terms) - ( sum of 24 terms)

                     =(3×(25)²/2 +(13×25)/2) - ( 3×(24)²/2 + (13×24)/2)

                     = 1100 - 1020

                     = 80

method 2

1st term = 8

sum of 2 terms = 19

2nd term = 19 - 8= 11

so. difference = 11- 8 = 3

25th term = (8 + (25 -1)×3) = 80