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[Math: Vectors] Clear my doubts please

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rickyponting66:
Hello,

Please see the question below, attachment.

answer to b) i) is

q-p
I did this like this PQ = OQ - OP  (position vectors)

next part ii) This is my concern

I do it like this,

OX = PX - PO   (position vectors)

so, it gets like this :
= q-p/3 - q
= q-p-3q/3
= -2q-p/3 (answer)

but on mark scheme, the answer is 2q+p/3

Whats wrong? and Whats weak in my concept of vectors, can any one please clear it?

The question is of M/J 2004 O Level Paper 02 Math 4024.

Dasith:

--- Quote from: rickyponting66 on May 30, 2011, 10:15:04 pm ---Hello,

Please see the question below, attachment.

answer to b) i) is

q-p
I did this like this PQ = OQ - OP  (position vectors)

next part ii) This is my concern

I do it like this,

OX = PX - PO   (position vectors)

so, it gets like this :
= q-p/3 - q
= q-p-3q/3
= -2q-p/3 (answer)

but on mark scheme, the answer is 2q+p/3

Whats wrong? and Whats weak in my concept of vectors, can any one please clear it?

The question is of M/J 2004 O Level Paper 02 Math 4024.
--- End quote ---

dude u sure u r ans 4 part 1 is correct ?

i get it like

i) (-p+q) / 3 = PX

ii)  P + (-p+q)= OX

yea ??

Saladin:
This needs to be handled in Subject Doubts.

Becca:
Dear Ricky, you actually did everything correctly... :) However, when you came to 'OX=PX-PO', I think that you accidentally substituted 'q' into 'PO' instead of 'p'. If you do this, you will get the answer...

To Darsith, I'm thinking that maybe you did something wrong in your calculations along the way, and forgot about the denominator of 3. I hope that I helped. :)

Dasith:

--- Quote from: Becca on September 04, 2011, 02:17:02 pm ---Dear Ricky, you actually did everything correctly... :) However, when you came to 'OX=PX-PO', I think that you accidentally substituted 'q' into 'PO' instead of 'p'. If you do this, you will get the answer...

To Darsith, I'm thinking that maybe you did something wrong in your calculations along the way, and forgot about the denominator of 3. I hope that I helped. :)

--- End quote ---

becca u knw that a vectors got both direction & size , we want to find PX right which is 1/3 of PQ
& we knw
 PQ=PO + OQ
 
 substitute :
PO=p &  OQ= q

To find PQ we go from P to O then from O to Q
the direction in PO is from O to P but we want to the other way around , so it becomes "-P"
OQ is pointing in the direction we want to so its "q"

when we find PQ its 1/3 of that to get PX

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