x² + y² = 9,左半圆为x = - √(9 - y²)令x = 3cosθ,y = 3sinθ,π/2 ≤ θ ≤ 3π/2dx/dθ = - 3sinθ,dy/dθ = 3cosθds = √[x'(θ)² + y'(θ)²] dθ = √(9sin²θ + 9cos²θ...