What is the sum of a 12–term arithmetic sequence where the last term is 13 and the common difference is –10?

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Answer 1 · 2016-09-30T13:45:38

Let's first determine the first term.

#t_n = a + (n - 1)d#

#13 = a + (12 - 1)-10#

#13 = a - 110#

#123 = a#

We now apply the formula #s_n = n/2(t_1 + t_n)#.

#s_12 = 12/2(123 + 13)#

#s_12 = 6(136)#

#s_12 = 816#

Hopefully this helps!