if s6=737.856 and r=4/5, what is the value of a1?

Answer 1

S_n = a₁(1 - rⁿ) / (1 - r) Formula from your textbook.

737.856 = a₁(1 - 0.8⁶) / (1 - 0.8)

Can you solve for a₁ and answer?

Answer 2

*** See Mark M's answer and please calculate the answer.

Often, it helps to see the data progression for a series (i.e., the sum) and the sequence. Here it is: