极坐标∫∫（x^2+y^2)^1/2dxdy=∫∫ r*r drdθ=∫[-π/2→π/2]dθ∫[0→2cosθ] r² dr=(1/3)∫[-π/2→π/2] r³ |[0→2cosθ] dθ=(8/3)∫[-π/2→π/2] cos³θ dθ=(8/3)∫[-π/2→π/2] cos²...