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Write i in trigonometric form. Since |i| = 1 and arg(i) = π/2, we have
i = exp(i π/2) = cos(π/2) + i sin(π/2)
By DeMoivre's theorem,
i² = exp(i π/2)² = exp(i π) = cos(π) + i sin(π)
and it follows that i² = -1 since cos(π) = -1 and sin(π) = 0.