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C4 Vectors....

June 2007 question 7c)

For some reason in the mark scheme it says



Why would this work? They only mention its an isoceles.
Also why cant you do

Then you would find

Which would give you the length of AB. I tried this but it doesnt work
or by their logic you could also do

And then do the same as above to work out the length, This doesnt work either..
(edited 7 years ago)
Reply 1
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Zacken
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Original post by fpmaniac
June 2007 question 7c)


Post a link to the question paper, please.
Reply 2
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fpmaniac
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Original post by Zacken
Post a link to the question paper, please.


Paper:
http://www.mrbartonmaths.com/resources/A%20Level%20Past%20Papers/Core%204/2007%20-%20June/C4AQAJune07QP.pdf

mark scheme:
http://www.mrbartonmaths.com/resources/A%20Level%20Past%20Papers/Core%204/2007%20-%20June/C4AQAJune07MS.pdf
Reply 3
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Zacken
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Original post by fpmaniac


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
Always draw a diagram with vectors :smile:

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
(edited 7 years ago)
Reply 5
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Zacken
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Original post by Katiee224
ps hi zacken


Hi. :tongue:
Reply 6
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fpmaniac
OP
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Original post by Katiee224
Always draw a diagram with vectors :smile:

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
Reply 7
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Zacken
22
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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