# Thread: move a point along a plane in 3d space

1. ## move a point along a plane in 3d space

I have a triangle in 3d space, and know all the angles and the coordinates of the 3 corners.
I also have a point on one of the edges
from that point I want to draw a perpendicular line, that follows the plane of the triangle- how would I do this mathematically

2. Originally Posted by ave
I have a triangle in 3d space, and know all the angles and the coordinates of the 3 corners. I also have a point on one of the edges from that point I want to draw a perpendicular line, that follows the plane of the triangle- how would I do this mathematically
I not at all sure that I understand what it is you require.

Is this it? Say that $A,~B,~\&~C$ are the three vertices of the triangle and point $
D \in \overline {AB}$
.
Now you want a line in the plane of the triangle through $D$ which is perpendicular to $
\overline {AB}$
.
If that is the case, the solution a straightforward.

Otherwise, you need to try again to explain the question.

3. Originally Posted by Plato
I not at all sure that I understand what it is you require.

Is this it? Say that $A,~B,~\&~C$ are the three vertices of the triangle and point $
D \in \overline {AB}$
.
Now you want a line in the plane of the triangle through $D$ which is perpendicular to $
\overline {AB}$
.
If that is the case, the solution a straightforward.

Otherwise, you need to try again to explain the question.
HI Plato- unfortunately my maths has a few holes when it comes to terminology and symbols, I can understand it quite easily, but have never studied it at uni, this unfortunately is my downfall, so I will try explain what I am looking for in picture form

the red dot will be a point that lies on the edge of a triangle- I need to draw a perpendicular line- (in green) from that point.

if you look in the side view (z/y axis), it must lie on the same plane as the triangle- hope that makes sense??

4. Then the equation of the line is $\ell (t) = D + t\left[ {\overrightarrow {AB} \times \left( {\overrightarrow {AB} \times \overrightarrow {AC} } \right)} \right]$.
Where the points are as I said above.
But if you are weak in mathematics that may not help you at all.