Chapter 11, Problem 10RE

Let ${\text{r}}_0=〈x_0,y_0,z_0〉\text{and}\text{r}=〈x,y,z〉.$ Describe the set of all points $\left(x,y,z\right)$ for which

$\left(\text{a}\right)\text{r}\cdot {\text{r}}_{\text{0}}=\text{0 }\left(\text{b}\right)\left(\text{r}-{\text{r}}_{\text{0}}\right)\cdot {\text{r}}_{\text{0}}\text{= 0}\text{.}$