极坐标转换：
∫∫ e^(x² + y²) dxdy
D
= ∫(0,π) ∫(0,2) re^(r²) drdθ
= (1/2)[θ] |(0,π) [e^(r²)] |(0,2)
= (π/2)(e⁴ - 1)