SOLUTION: Find the sum of first 15 terms of an AP., whose nth term is 3-2n.

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Question 998264: Find the sum of first 15 terms of an AP., whose nth term is 3-2n.
Answer by mananth(16946) About Me  (Show Source):
You can put this solution on YOUR website!

nth term = 3-2n
t1= 3-2=1
t2= 3-4=-1
t3=3-6 = -3
The AP is 1,-1,-3 .........

here a = 1, d= -2 , n=15
Sn = +%28n%2F2%29%282a%2B%28n-1%29d%29
S 15 = +%2815%2F2%29%282%281%29%2B%2815-1%29%2A%28-2%29%29
S15 = %2815%2F2%29+%2A+-26
= -13*15
=-195