Posted by **emi** on Saturday, June 7, 2014 at 2:30pm.

the sum of 11terms of A.P is 891.find the 28th and 45th terms if the common difference is 45

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**Steve**, Saturday, June 7, 2014 at 4:26pm
If we add up terms n through n+10, then we get

(n+10)/2 (2a + (n+9)45) - (n-1)/2 (2a+(n-2)45) = 891

11a+495n = -1089

a+45n = -99

If we are adding up the first 11 terms, then a = -144 and

T28 = -144+27*45 = 1071

T45 = -144+44*45 = 1836

But, if we add up terms T5 through T15, then n=5, and a = -324 and

T28 = 891

T45 = 1656

Better tie down your conditions a bit better

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