Recall I
line in 3-D Define $\vec r_0$ and $\vec r$ are position vectors of $P_0$ and $P$, and $\vec a$ represents $\vec{P_0P}$. if $\vec v$ is parallel to $\vec a$, then: $\vec{r} = \vec{r_0} + \vec{a}$ $\vec{a} = t\vec{v}$ $\vec{r} = \vec{r_0} + t\vec{v}$ in othe words, $(x_r, y_r, z_r) = (x_{r_0}, y_{r_0}, z_{r_0}) + t \cdot (x_v, y_v, z_v)$ sphere the sphere’s center point is $(x_0, y_0, z_0)$ radius = $r$ all point on surface: $(x, y, z)$ $(x - x_0)^{2} + (y - y_0)^{2} + (z - z_0)^{2} = r^2$...