A.9. Express the following in the Σ notation: a. x₁ + x₂ + x3 + x4 + x5 b. x₁ + 2x2 + 3x3 + 4x4 + 5x5 c. (x² + y²) + (x² + y²) + (x² + y²) + ... + (x² + y ² )