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1. URGENT! exam tomorrow ! vector equation of a line
coordinates:

a( 1,2,2) b: (0,0,2)

find the vector: ---->
AB

and the vector equation of the line AB

I thought the answer is (1,2,2) + lander(-1,-2,0)

but infact it is (0,0,2) + lander(1,2,0)

i dont understand, i thought you start form the starting point which would be A, not the other way around?

this is from the mei c4 june 09 paper question 7
paper can be downloaded here:
http://www.mei.org.uk/index.php?section=papers
2. Re: URGENT! exam tomorrow ! vector equation of a line
(Original post by 2cool)
coordinates:

a( 1,2,2) b: (0,0,2)

find the vector: ---->
AB

and the vector equation of the line AB

I thought the answer is (1,2,2) + lander(-1,-2,0)

but infact it is (0,0,2) + lander(1,2,0)

i dont understand, i thought you start form the starting point which would be A, not the other way around?

this is from the mei c4 june 09 paper question 7
paper can be downloaded here:
http://www.mei.org.uk/index.php?section=papers
its the same thing.
3. Re: URGENT! exam tomorrow ! vector equation of a line
When calculating vectors, it always the reverse. So:

AB = b-a
4. Re: URGENT! exam tomorrow ! vector equation of a line
DavidMRoper is right

(1,2,2) + lamda(-1,-2,0) = (1,2,2) - lamda(1,2,0)

So the sign of the constant lamda for each specific point on the line will just be reversed

Example:

(0,0,2) = (1,2,2) +1(-1,-2,0) = (1,2,2) -1(1,2,0)

In fact, you could use any scalar multiple of (1,2,0) and the answer would still be correct.
5. Re: URGENT! exam tomorrow ! vector equation of a line
(Original post by thefifthfoo)
DavidMRoper is right

(1,2,2) + lamda(-1,-2,0) = (1,2,2) - lamda(1,2,0)

So the sign of the constant lamda for each specific point on the line will just be reversed

Example:

(0,0,2) = (1,2,2) +1(-1,-2,0) = (1,2,2) -1(1,2,0)

In fact, you could use any scalar multiple of (1,2,0) and the answer would still be correct.
weird... I thought it was called lander for the last 2 years.. and i always think it looks like some kind of moon lander :L

but thanks for the help all, cant believe i didnt know this when i can do some of the complicated stuff!
6. Just remember a line is a line. You have one point on it and some vector to move across it. Kinda like having a car and placing it in a road - doesn't matter where you start or, as long as the direction is right

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