Help please??

Question

Help please?? <3
Find S12 for 1 + 6 + 11 + 16 +…

308

322

336

342

hartnn · Answer

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

anonymous · Answer

no

hartnn · Answer

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

anonymous · Answer

how do you find the common difference?

hartnn · Answer

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?

anonymous · Answer

so the common difference is 5

hartnn · Answer

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

anonymous · Answer

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

hartnn · Answer

yup, go on!

anonymous · Answer

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

hartnn · Answer

so whats your final answer ?

anonymous · Answer

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

hartnn · Answer

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

anonymous · Answer

oh okay so it 342

hartnn · Answer

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