In Figure 1.5.9(a), the vectors A and B lie in the plane determined by the points Q, R, and S. (The positions of A and B have been reversed with respect to their locations in Figure 1.5.8(a).)

The blue vector is $\mathbf{A}\times \mathbf{B}$, and is therefore normal to the plane. The dotted red line from the tip of C (which corresponds to point P) is parallel to the plane and delineates the projection of C onto $\mathbf{A}\times \mathbf{B}$.

The distance from point P to the plane is the length of the projection of C onto $\mathbf{A}\times \mathbf{B}$ , a length designated by $d$, the leg of a right triangle whose hypotenuse is $∥\mathbf{C}∥$.

Therefore, $d\=∥\mathbf{C}∥$ $\cos \left(\mathrm{\θ}\right)$, where $\mathrm{\θ}$ is the angle between C and $\mathbf{A}\times \mathbf{B}$.