What is the sum of a 12-term arithmetic sequence where the last term is 13 and the common difference is –10?
605
660
748
816

Question

What is the sum of a 12-term arithmetic sequence where the last term is 13 and the common difference is –10? 
605
660
748
816

saifoo.khan · Answer

Do you know the sum formula?

saifoo.khan · Answer

$$S_t = \frac{n}{2} (a+l)$$Where a is the forst term.
L is the last.  
n is the term of total terms.

saifoo.khan · Answer

i meant,
n is the number of terms.

anonymous · Answer

saifoo in which grade u r now?

saifoo.khan · Answer

@rockstar111 , heard of A-levels?

saifoo.khan · Answer

Wait! you can use another formula as well. xD

anonymous · Answer

A-LEVELS what is this?

saifoo.khan · Answer

These are two formulas, http://intmstat.com/series-binomial-theorem/Image1702.gif Use second one.

campbell_st · Answer

ist you need to find the value of the1st  term

using the last term information, and common difference

$$13= a + (12 -1) -10$$

$$13 = a- 110$$

a = 123


then you can find the sum 

$$s _{n}= \frac{n}{2}(a + l)$$

saifoo.khan · Answer

@rockstar111  Let's head to chat and do there.

anonymous · Answer

i think its 816, am i right?

campbell_st · Answer

thats it