∫[0,∞]e^(‐x^2)dx ∫[0,∞]e^(‐y^2)dy
∫∫e^‐(x^2+y^2)dxdy
∫∫e^‐(r^2)drcost^drsint
(costdr-rsintdt)^(sintdr+rcostdt)=-rsintsintdtdr+rcotcostdrdt
=rdrdt
∫∫e^‐(r^2)rdrdt=-pie^-r^2/2=-pi/2e^-r^2|0->1=pi/2