a =a . r n-1. a =1 n 1 1 a =a .r 5-1. a =1.3=3 5 1 2 a =a . r 5-1. a =3.3=9 5 1 3 81=1. r 4. a =9.3=27 4 81/1=r 4. a =27.3=81 5 81 1/4 =r. 3=r