can u plz help me solve this 2 question in the Description thank you

Question

1.Find the sum of the first 3 terms of a geometric sequence if a = 5 and r = 3.

2.Find the third term in a geometric sequence if a = 8 and r = -1.

Answer 1

  Sn = SUM [ 5(3)^n ]

 

  5(3)^1 + 5(3^2) = 5(3^3) 

 = 5*3 + 5*9 + 5*27

 = 5 ( 3+9+27)

 = 5 ( 39)

 =  195

 

 

 

Xn = 8(-1)^n

   X1 = 8(-1)^1 = -8

   X2 = 8(-1)^2 = 8

   X3 = 8(-1)^3 = -8