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# closed Watch

1. closed
2. (Original post by lj789)
Topic: Vector

prove if 2 straight lines, r = a + ku, r = b + hv intersect, then v*(b X u) = v*(a X u)

b X u is a cross product
* means dot product

any help is appreciated thanks
Since you know the two lines meet the obvious thing is to specify their point of intersection in terms of the two lines and equate.

Then perform the appropriate vector and dot multiplications to get into the desired form. Note that some terms will evaluate as zero.

Have a go, and post your working if you're still stuck.
3. (Original post by ghostwalker)
Since you know the two lines meet the obvious thing is to specify their point of intersection in terms of the two lines and equate.

Then perform the appropriate vector and dot multiplications to get into the desired form. Note that some terms will evaluate as zero.

Have a go, and post your working if you're still stuck.
del-
4. If they intersect, then you know that there are values for h and k such that the two vectors are the same.

That is, you know that a+ku = b+hv (even though you don't know what h and k are).
5. del-
6. del-
7. You don't need to know what h and k are to answer the question.
8. del-
9. Crossing both sides with u looks like a good place to start...
10. Wrong post pls delete
11. del-
12. Note that uXu=0 and (vXu)*v=0.
13. (Original post by allylalcohol)
Note that uXu=0 and (vXu)*v=0.
problem solved, thanks!
14. (Original post by DFranklin)
Crossing both sides with u looks like a good place to start...
solved now, thanks a lot!

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