[Math] Analytic Geometry: line parallel to a plane

analytic geometry

I'm trying to figure out how to solve this problem of Analytic geometry of the space:


I have a line described by the system:
$$\begin{cases}
2x-y+z-1 &=0\\
5x+3y-8& =0
\end{cases}$$

and I have to verify wether this line is parallel or not to the following plane:

$x-y+z+10=0$


I tried to make the reduced equations of the line and then swap the $z$ and $y$ I obtained into the plane equation.

Solved the equation with variable $x$ but then I got 3 solutions which don't seem to be the real point of intersection between the line and the plane.


Any idea of how should I solve this problem?

Best Answer

The answers given are shorter, but you could also proceed in the following way: you are given a line (two equations) and a plane (one equation) and in $\mathbb{R}^3$ there are only three possibilities: the line is parallel to the plane and not a part of it, the line intersects the plane or the line is parallel to the plane and a part of it.

Looking at your equations, we can consider them as one system of equations. These systems have either no, one or infinitely many solutions! This corresponds to the previous situations I have described. Using this approach, we find the following system of equations: $$\begin{cases} 2x - y + z &= 1\\ 5x + 3y &= 8\\ x - y +z &= -10 \end{cases}$$ which you can put in an augmented matrix and row reduce. This gives you the following solution: $(x;y;z) = (11; -15,66666 ; -35,66666)$ (where I used $;$ to separate the coordinates and $,$ to denote the decimal parts). Hence we find one solution, and so the line must intersect the plane.

To show you that the line does intersect, I have plotted this in GeoGebra using the 3D option and found an intersection piont $A$ with the coordinates I mentioned: enter image description here