Find a formula in a, r, m, and n for the sum arᵐ + ar^(m+1) + arᵐ+² + ... + arᵐ+ⁿ where m and n are integers, n ≥ 0, and a and r are real numbers. Justify your answer.
a) Sₙ = a * rᵐ(1 - rⁿ+¹) / (1 - r)
b) Sₙ = a * (1 - rᵐ+ⁿ+¹) / (1 - r)
c) Sₙ = a * (rᵐ+ⁿ+¹ - 1) / (r - 1)
d) Sₙ = a * (1 - rᵐ+¹) / (1 - r)
