I'm looking for the sum of a 12-term sequence where the last term is 13 and common difference of -10. How do I find the first term so I can solve?
What I have so far is ___= 12/2 (___+(11)-10).

Question

I'm looking for the sum of a 12-term sequence where the last term is 13 and common difference of -10. How do I find the first term so I can solve? 
What I have so far is ___= 12/2 (___+(11)-10).

anonymous · Answer

$$a+11*(-10)=13$$ 
is the equation,solve for a.

anonymous · Answer

a + 11(-10) = 13

a = 123

(12/2)(123+(11)-10) = 744

anonymous · Answer

But the answer can only be one of the following:

-->  605 
-->  660
-->  748
-->  816

anonymous · Answer

equation for sum has 2*a instead of a

anonymous · Answer

u will get 816

anonymous · Answer

Wait, if a=123
2a= 246

The sum would come out too high.

anonymous · Answer

(12/2)(2*123+(11)-10)
 =6(246-110)=816

anonymous · Answer

last term(L)= a + (n-1)*d....

anonymous · Answer

I think I may have entered it into my calculator wrong..

Thanks.