math
posted by emi on .
the sum of 11terms of A.P is 891.find the 28th and 45th terms if the common difference is 45

If we add up terms n through n+10, then we get
(n+10)/2 (2a + (n+9)45)  (n1)/2 (2a+(n2)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