Posted by Melissa on .
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

Math 
Steve,
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