C4 Dot Product

So (like in the question below) you would use the dot product when dealing with the "direction vectors"

but then you would use the cross product when with a line equation multiplied with another vector etc..

So what are the do's and don'ts when using the dot product

C4 questions never require the cross-product - it's a further maths topic.

I don't really understand your question. You can dot product any two vectors and why you would depends on what you're trying to do. If a question is realated to a perpendicular property then the dot product will probably be involved somewhere.

Dot product is in the spec for C4 edexcel maths.

My question is that when dealing with the dot product would you only use the "direction" of the vector or?

I know it is. But the "cross product", which you mentioned in your first post is not part of C4.


You mean the direction vector of the line.

To show that two lines are perpendicular, you can show that their two direction vectors are perpendicular. Remember that a vector equation of a line is r=a+tb where a is the position vector of a point on the line.

Showing that the position vectors of points on two lines are perpendicular would not show that the lines are perpendicular.

So in summary, when you have vector equations of lines, the most common dot product operation that you will do will be with the direction vectors. But you may dot product something else.

Oh yeh I already understand the whole perpendicular concept but my question was (sorry I should've made this clear):

A lies on L1
B lies on L2

You don't know what exact vector B is but you do know the equation of L2.

So you can therefore do the dot product with A . L2 = 0.

But my main question is in the situation we used the full like equation of L2, not just the direction vector of the line. Therefore, is the dot product suitable for full line equations and a direction vector of a line?

A lies on L1
B lies on L2

You don't know what exact vector B is but you do know the equation of L2.

So you can therefore do the dot product with A . L2 = 0.

After reading these posts, I do not understand the objective. If it helps, the scalar product equals 0 when two vectors are perpendicular. In this question, I am uncertain what is being asked although, so find out if they are perpendicular, you must first solve to find s and t. Then apply the scalar product rule.

Sorry, my explanation was not clear whatsoever. Forget what I said above.

My question is this:

When using the dot product can you only use the direction vector of the line?

Yes.

Sorry, my explanation was not clear whatsoever. Forget what I said above.

My question is this:

When using the dot product can you only use the direction vector of the line?