Answer:
A is the answer.
Step-by-step explanation:
You are being asked which one is NOT the correct answer. C is the standard formula for the first n terms. It is a correct answer.
Anything that says Sum = a1 + a2 + a3 + a4 ... an then it is the correct answer.
So far B and C are correct. D has a small error in it. It should be i = 1 not i - 1. That's a very easy mistake to make and for the moment I will see if A is correct.. Then we'll revisit D.
A has an + a1 + (n - 1)*d in it but a1 + (n - 1)* d is the last term of the subgroup of the infinite series. So what you have in answer A is an + an = 2an which is the sum of nothing.
D is fine. Just correct the typo.
Answer: A is wrong
