Question

The formula to find a certain number in an arithmetic sequence is an=a1+d(n−1) .

Solve for n.

an−a1/d+1=n

1−d/an+a1=n

1−an−a1/d=n

an+a1/d+1=n

Answer 1

(an -a1)/d +1 = n

Explanation:

Undo what is done to n, in reverse order. When evaluating this expression, you ...

• subtract 1 from n
• multiply the difference by d
• add a1

When you undo these, you get ...

... an -a1 = d(n-1) . . . . . . subtract a1

... (an -a1)/d = n - 1 . . . . divide by d

... (an -a1)/d +1 = n . . . . add 1

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Comment on answer list

Please note the use of parentheses in the answer here. These identify that the difference (an -a1) is what is divided by d. None of the offered answers has these parentheses, nor are they typeset in a way that would indicate the intent to divide (an -a1) by d. As given, none of the offered answers is correct.