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If an arithmetic progression the sum of first n term is 3n square divided 2plus 13n by,find the 25th term

Posted by Shareef Khan 6 years, 10 months ago

CBSE > Class 10 > Mathematics

1 answers

Yogita Ingle 6 years, 10 months ago

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

1Thank You

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