Can anyone explain to me what this is asking/how to do this? i think it's for the summation formula but if i'm given a1, an, and n, what are the other numbers for?

D 3500

Step-by-step explanation:

This question is asking for the sum of all the terms in an arithmetic series.

So using the formula : Sn = n/2 (a+l) , where Sn is the sum of n terms, a is first term, and l is last term, we try to find Sn, with n being 25, a being 20, and an which is a25 or the twenty fifth term being 260, also being the last term.

Hence we can produce the formula for Sn which is Sn=25/2 (20+260)

Sn=25/2 (280)

=3500

b

step-by-step explanation:

a difference is the result of subtraction. while each of these expressions has subtraction in it, a difference will be the result when subtraction is the last operation applied in the order of operations for the expression. this means subtraction must be outside of the parenthesis. only b has this.