Math
posted by Melissa .
The 13th termof an arithmetic sequence is 61 and the 20th term of the squence is 82. Calculate the sum of the first 25 terms of the sequence

T13 = a+12d = 61
T20 = a+19d = 82
so,
T20T13 = 7d = 21
d = 3
a = 25
S25 = 25/2 (25 + 25+24*3)
= 25/2 (122) = 25*61 = 1525