Asked by Merty

The following problem refers to an arithmetic sequence.

If a_4 = 15 and a_10 = 39, find a_40 and S_40.

The _ represents subscript
Answered by Reiny
term4 = a+3d = 15
term10= a+9d = 39
subtract them
6d = 24
d = 4
in a+3d = 15
a + 12 = 15
a = 3

term40 = a+39d = ...
sum40 = (40/2)(2a + 39d) = ...
Answered by Anonymous
Thanks - I had a wrong calculation in the first two term problems that gave me the wrong "d", giving me the wrong answers. Thanks for the help.
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