I have a study guide from my teacher and it is on arithmetic series. I'm pretty confident in this area and I think I have the question right. She had a different answer tho. Can you guys check it out and tell me if my answer is right or if hers is.I will work out the problem.

What is the sum of the arithmetic sequence 5,913... if there are 36 terms?

I used the formula sn=n/2 [2a1 + (n-1)d]

a1=5
an=
n=36
d=4
sn=?

sn=36/2 [2(5)+(36-1)4]
sn=36/2 [2(5)+(35)4]
sn=36/2 [2(5)+140]
sn=36/2 [25+140]
sn=36/2 [165]
sn=18(165)
sn=2,970

my answer 2970, my teachers answer was 2,700

Question

I have a study guide from my teacher and it is on arithmetic series. I'm pretty confident in this area and I think I have the question right. She had a different answer tho. Can you guys check it out and tell me if my answer is right or if hers is.I will work out the problem.

What is the sum of the arithmetic sequence 5,913... if there are 36 terms?

I used the formula sn=n/2 [2a1 + (n-1)d]

a1=5
an=
n=36
d=4
sn=?

sn=36/2 [2(5)+(36-1)4]
sn=36/2 [2(5)+(35)4]
sn=36/2 [2(5)+140]
sn=36/2 [25+140]
sn=36/2 [165]
sn=18(165)
sn=2,970

my answer 2970, my teachers answer was 2,700

phi · Answer

I did not use your formula, but I came up with 2700.

anonymous · Answer

But I don't understand, did I use the formula correct?

blacksteel · Answer

Your answer is off because 2*5 is not equal to 25

phi · Answer

Blacksteel found your (small) mistake.  You did use the formula correctly, you multiplied wrong

anonymous · Answer

Thank you! For some reason I was thinking 2*5 was 5^2. Now I get it.

phi · Answer

We call that having a brain spasm.