Answer:
See below.
Step-by-step explanation:
Call the sum of the first n terms S.
S = a + ar + ar^2 + ar^3 + ... + ar^(n -1)
Multiply both sides by r.
Sr = ar + ar^2 + ar^3 + ar^4 + ... + ar^n
No subtract S - Sr.
S = a + ar + ar^2 + ar^3 + ar^4 + ... + ar^(n -1)
(-) Sr = ar + ar^2 + ar^3 + ar^4 + ... + ar^(n - 1) + ar^n
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S - Sr = a - ar^n
S(1 - r) = a(1 - r^n)
S = a(1 - r^n)/(1 - r)
S = a(r^n - 1)/(r - 1)
a + ar + ar^2 + ar^3 + ... + ar^(n -1) = a(r^n - 1)/(r - 1)
