I've spent so long trying to figure this out and I just CAN'T. I can see the relationship but I don't know how to actually calculate this through
Sn = u1(r^n-1) / 4-1
or Sn = n/2(2u1 + (n-1)d)
I tried calculating them both as geometric and as arithmetic but I can't figure out a consistent d or r?
Can someone help me please?