1 + x + x^2 + + x^n
b₁ = 1
q = x
1. q=x<1
S = b₁/(1 - q) = 1/(1 - x)
2. q=x>1
S = b₁*(q^n - 1)/(q - 1) = (x^n - 1)/(x - 1)
1 + x + x^2 + + x^n
b₁ = 1
q = x
1. q=x<1
S = b₁/(1 - q) = 1/(1 - x)
2. q=x>1
S = b₁*(q^n - 1)/(q - 1) = (x^n - 1)/(x - 1)