# arithmetic

What is the sum to 15 terms of an arithmetic progression whose 8th term is 4?

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1. W.k.t,
Sn = n/2 {2a + (n-1)d} then
S15 = 15/2 {2a + 14d}
=15/2 {2(a+7d)}
wkt, a+7d = 15
S15 = 15(15)=225

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posted by subbu

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