x=ρcosθ,y=ρsinθ.故由ρ=6/(1+2cosθ).===>ρ(1+2cosθ)=6.===>ρ+2ρcosθ=6.===>ρ=6-2x.===>ρ²=(6-2x)².===>x²+y²=4x²-24x+36.===>3x²-y²-24x+36=0.===>(x-4)²/4-y²/12=1.