## anonymous one year ago Help please?? <3 Find S12 for 1 + 6 + 11 + 16 +… 308 322 336 342

1. hartnn

so you know the formula for Sum of terms in Arithmetic series?

2. anonymous

no

3. hartnn

ok, here you go! $$\Large S_n = n/2 (2a+ (n-1)d)$$ where a = First term = 1 d = common difference = .... ? can you find? and n = number of terms = 12, because we need sum of 12 terms

4. anonymous

how do you find the common difference?

5. hartnn

d = common difference = difference between next term and current term.. like d = 2nd term - 1st term = 3rd term - 2nd term = ..and so on now can you find the d?

6. anonymous

so the common difference is 5

7. hartnn

thats right! d= 5 :) now plug in all the values in the formula :)

8. anonymous

$s _{12}=12/2(2\times1+(12-1)5)$ Right?

9. hartnn

yup, go on!

10. anonymous

$s _{12}=12/2(2\times1(11)5)$

11. hartnn

12. anonymous

i got 12/114 but that's not an option so... I don't know what I did wrong?

13. hartnn

12/2 is separate which will evaluate to 6 6 (2+ 55) = ... ?

14. anonymous

oh okay so it 342

15. hartnn

yes! thats correct :) the whole $$2a + (n-1)d$$ is in numerator and not in denominator :)