Posted by Melissa on Saturday, December 17, 2011 at 12:46am.
T13 = a+12d = 61
T20 = a+19d = 82
so,
T20-T13 = 7d = 21
d = 3
a = 25
S25 = 25/2 (25 + 25+24*3)
= 25/2 (122) = 25*61 = 1525
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