For each of the following quadratic forms, determine if it is positive definite, negative definite, positive semidefinite, negative semidefinite, or indefinite.
A. f(x₁ , x₂ , x₃) = (x₁)²
B. f(x₁ , x₂ , x₃) = (x₁)² + (2x₂)² + x₁ x₃
C. f(x₁ , x₂ , x₃) = (x₁)² + (x₃)² + 2x₁ x₂ - 2x₁ x₃ + 2x₂ x₃