Question 760501
believe it or not i am answering my own question!
a) fill in the Sn formula for both given s subscript 3, and then rearrange to get a subscript 1 on one side.
b) fill in the Sn formula for s subscript 6 and then substitute the above for a subscript 1.
c) rearrange the expression to get the r's on one side.  rememeber 1-r^6 is = to (1-r^3) (1+r^3).  use this to cancel.
d) minus 1 on each side
e) take the cube root to get r
f) substitue this in the expression for a subscript 1 to get a subscript 1.