Find the sum of the first 4 terms of a geometric sequence for which a1=1 and r=2. Sum= ?

Answer 1

In case of Geometric Series:

S₄ = a₁ + r*a₁ + r² *a₁ + r³ *a₁

Answer 2

a1 = 1

a2 = 1(2) = 2

a3 = 1(2^2= 4

a4 = 1(2^3 = 8

an = a1(2)^n-1

a4 = 1(2)^4-1 = 2^3 = 8

first 4 terms are 1,2, 4, 8

sum = 1+2+4+8 = 15

Sn = a1(r^n-1)/r-1) r=2, a1 = 1

n= 4

S4 = 1(2^4 -1)(2-1) = 16-1/1 = 15