Question

Each of these sequence or series formulas involves four quantities, for example, a1, an, d, r, n, and Sn.

For each formula, describe the four quantities. Then explain how you can find the fourth quantity if you know the values of the other three.
(A. a subscript n=a subscript 1+(n-1)d
(b. s subscript n=n/2(a subscript 1 + a subscript n)
(c. a subscript n = a subscript 1r^n-1
(d. s subscript n = a subscript 1 (1-r^n)/1-r
I am so lost please help...