We say that a line in the direction of a vector is parallel to a plane if is parallel to . Consider the line through the point and parallel to the vector . Determine whether is parallel to the following planes.

- The plane through the point and spanned by and .
- The plane through the points .
- The plane determined by the Cartesian equation .

- This asks if is in the span of , i.e., does there exist such that
From the second equation we have . Then, from the first, which implies

But then,

Thus, there is no solution, so the line is not parallel to the plane.

- The plane through the points is the set of points
For to be in the span of we must have such that

From the first equation we have . Then from the second we have which implies

But then,

Hence, is not parallel to .

- The plane with Cartesian equation is the set of points
The points are all in . So,

Thus, we ask if is in the span of . This requires that there exist such that

But, this fails since the second and third equations implies and . But then

Hence, this line is not parallel to the plane.