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
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) = ...