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# C4 Vectors.... Watch

1. June 2007 question 7c)

For some reason in the mark scheme it says
$AP^2 = 504$
$AB^2 = 2AP^2$

Why would this work? They only mention its an isoceles.
Also why cant you do
$\overrightarrow{OA} + 2\overrightarrow{AP} = \overrightarrow{OB}$
Then you would find
$|\overrightarrow{OB} - \overrightarrow{OA}|$
Which would give you the length of AB. I tried this but it doesnt work
or by their logic you could also do
$\overrightarrow{OP} + \overrightarrow{AP} =\overrightarrow{OB}$
And then do the same as above to work out the length, This doesnt work either..
2. (Original post by fpmaniac)
June 2007 question 7c)
3. (Original post by Zacken)
Paper:
http://www.mrbartonmaths.com/resourc...QAJune07QP.pdf

mark scheme:
http://www.mrbartonmaths.com/resourc...QAJune07MS.pdf
4. I think that the equation AP = PB is just referring to the lengths of AP and PB and not the actual vectors themselves, hence why you can't derive those equations you did in the OP.

Anywho, if you look at the sketch below (remember that the lines are perpendicular)

then the triangle APB forms a right angled triangle with AP = PB.

So using pythagoras: AB^2 = AP^2 + PB^2 = AP^2 + AP^2 = 2AP^2
5. Always draw a diagram with vectors

Allot of the time you end up with some sort of shape such as a triangle or something and then you can just apply trigonometry to find the answer

ps hi zacken
6. (Original post by Katiee224)
ps hi zacken
Hi.
7. (Original post by Katiee224)
Always draw a diagram with vectors

Allot of the time you end up with some sort of shape such as a triangle or something and then you can just apply trigonometry to find the answer

ps hi zacken
(Original post by Zacken)
I think that the equation AP = PB is just referring to the lengths of AP and PB and not the actual vectors themselves, hence why you can't derive those equations you did in the OP.

Anywho, if you look at the sketch below (remember that the lines are perpendicular)

then the triangle APB forms a right angled triangle with AP = PB.

So using pythagoras: AB^2 = AP^2 + PB^2 = AP^2 + AP^2 = 2AP^2
Thanks.

Now, having looked back at all the papers, they dont seem to use the simple vector method (OA + BA ) etc, for triangles. Will probably need to have another look at this
8. (Original post by fpmaniac)
Thanks.

Now, having looked back at all the papers, they dont seem to use the simple vector method (OA + BA ) etc, for triangles. Will probably need to have another look at this
No problem.

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