1). 3, 9, 27,.......... S5=? 2). 625, 125, 25,........ S4=?

1.3,9,27
  a=3
  r=u2/u1=9/3 = 3

  s5 ..?
  s5=a(rⁿ-1)/r-1
  s5=3(3⁵-1)/3-1
  s5=3(242)/2
  s5=726/2
  s5=363

2.)625,125,25,..
   a=625
   r=u2/u1=125/625 = 0,2
  s4 ...?

  s4=a(1-rⁿ)/1-r
  s4=625(1-(0,2)²)/1-0.2
  s4=625(0,96)/0,8
  s4=600/0,8
  s4=750