What is the sum of the geometric sequence 1, 4, 16, if there are 8 terms?
21,845
43,690
65,535
87,380

The sum of geometric sequence:

S n = a 1 * ( q^n - 1 ) / ( q - 1 )

where:  a 1 = 1,

q = a 2 : a 1 = 4 : 1 = 4

S 8 = 1 * ( 4^(8) - 1 ) / ( 4 - 1 ) =

= ( 65,536 - 1 ) / ( 4 - 1 ) = 65,535 / 3 = 21,845