Question

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

Answer 1

a12 is 12th term
a is 1st term
13=a+(12-1)*(-10)
13=a-110
a=123
S12= {1/2}*(a+a12)
=0.5*(123+13)
=0.5*136
=68

Answer 2

1) 605
2) 660
3) 748
4) 816
these are the given answers

Answer 3

ok so where he messed up was in the second part the equation is : S_n=n/29a_1+a_n
So
S_n=12/2(123+13)
S_n=6(136)
S_n=816

Answer 4

so its d

Answer 5

oh! that's what i put! great thank you!!

Answer 6

no problem!