12. The vertex of a surface of revolution that is conical coincides with the origin $O$ of a rectangular coordinate system $O x y z$, the axis of rotation coincides with the positive direction of the $O z$ axis, and the ray, by the rotation of which the surface is obtained, forms an angle $\varphi$ with the $O z$ axis. Write the equation of this surface in coordinates.
12. Let the point $M(x ; y ; z)$ be at a distance $r$ from the $O z$ axis. Then $x^{2}+y^{2}=r^{2}$, since the point $M^{\prime}$ - the projection of $M$ on $O x y$ is at a distance $r$ from the point $O$. But $r=z \operatorname{tg} \varphi$. Therefore, the equation of the conical surface will be the relation $x^{2}+y^{2}$ $-z^{2} \operatorname{tg}^{2} \uparrow=0$.