What is the sum of the first 18 terms of the arithmetic series
11 + 15 + 19 + 23 + ...?
A) 684
B) 810
C) 831
D) 846
If anyone could help, that'd be really appreciated. Please include step by step intructions. Thank you

Question

What is the sum of the first 18 terms of the arithmetic series
11 + 15 + 19 + 23 + ...?
A) 684
B) 810
C) 831 
D) 846 
If anyone could help, that'd be really appreciated. Please include step by step intructions. Thank you

anonymous · Answer

http://www.regentsprep.org/regents/math/algtrig/atp2/ArithG3.gif

one098 · Answer

Well each number increases by 4, so keep adding 4, until you reach an 18th number, then add them all together.

anonymous · Answer

use the formula Sn = n(a1+an)/2

anonymous · Answer

I tried that and didn't get anywhere close to the answers provided. I must've done something wrong or something and how would I use the formula?

one098 · Answer

$$11+15+19+23+27+31+35+39+43+47+51+55+59+63+67+71+75+79$$

one098 · Answer

The last number is 79.

one098 · Answer

Put all those numbers into a calculator.

anonymous · Answer

AHHH, I got it now. Thank you, I must've been doing something wrong since I got a bigger number but now I got it. I appreciate it, thank you once again

one098 · Answer

You're welcome!!

anonymous · Answer

you also use the formula an= a1 + (n-1)d, in order to find an
then use the first formula i gave you
Sn is what your finding(the sum) n=# of terms(18) a1= first term, an=nth term

anonymous · Answer

one098's way is too long

anonymous · Answer

|dw:1423105657402:dw|

anonymous · Answer

thnx @One098

anonymous · Answer

did this help?

anonymous · Answer

Yes thank you! That's a much simpler way and it was step by step so I understood it well. Thank you!

anonymous · Answer

yw cx