发帖总被删，也是服了

{
ParametricPlot3D[{x,y,z}/.Solve[{x^2+y^2==z^2,z-z0-y Tan[θ]==0}/.{z0->1/2,θ->Pi/3},
{y,z}]//Evaluate,{x,-2,2}],
ParametricPlot3D[{Sqrt[1/8]Tan[t],1/4 (-Sqrt[3]+Sec[t]),1/4 (-1+Sqrt[3] Sec[t])},{t,0,2Pi}]
}
{
ParametricPlot3D[{x,y,z}/.Solve[{x^2+y^2==z^2,z-z0-y Tan[θ]==0}/.{z0->1/3,θ->Pi/6},{y,z}]//Evaluate,{x,-2,2}],
ParametricPlot3D[{Sqrt[1/6]Cos[t],1/6 (Sqrt[3]-3 Sin[t]),1/6 (3-Sqrt[3] Sin[t])},{t,0,2Pi}]
}

分段表示
If[Tan[θ]^2<1,{(z0 Cos[t])/Sqrt[1-Tan[θ]^2],(z0 (Tan[θ]+Sin[t]))/(1-Tan[θ]^2),(z0 (Tan[θ] Sin[t]+1))/(1-Tan[θ]^2)},{(z0 Tan[t])/Sqrt[Tan[θ]^2-1],(z0 (Sec[t]-Tan[θ]))/(Tan[θ]^2-1),(z0 (Tan[θ] Sec[t]-1))/(Tan[θ]^2-1)}]

……我觉得这个问题值得去SE一问。