If x = r cos θ and y = r sin θ, then x² + y² equals:
Correct: B
Substitute the given expressions for x and y into x² + y²: x² + y² = (r cos θ)² + (r sin θ)². This expands to r² cos²θ + r² sin²θ. Factor out r²: r²(cos²θ + sin²θ). We know the identity cos²θ + sin²θ = 1. So, the expression simplifies to r² × 1 = r².