Sum of the finite geometric series $∑_{i=0}^{n-1} x^i$.

Sum of the finite geometric series $∑_{i=0}^{n-1} x^i y^{n-1-i}$.

$x^n-y^n=(x-y)∑x^ky^{n-1-k}$ reformulated without - signs.

$x^n-y^n=(x-y)∑x^ky^{n-1-k}$ reformulated without - signs.