IGCSE/GCSE/O & A Level/IB/University Student Forum

Qualification => Subject Doubts => GCE O - Level => Math => Topic started by: rickyponting66 on May 30, 2011, 10:15:04 pm

Title: [Math: Vectors] Clear my doubts please
Post by: 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.

Title: Re: [Math: Vectors] Clear my doubts please
Post by: Dasith on June 01, 2011, 11:29:51 am

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.

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 ??

Title: Re: [Math: Vectors] Clear my doubts please
Post by: Saladin on September 03, 2011, 10:33:44 pm

This needs to be handled in Subject Doubts.

Title: Re: [Math: Vectors] Clear my doubts please
Post by: 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. :)

Title: Re: [Math: Vectors] Clear my doubts please
Post by: Dasith on September 05, 2011, 05:51:44 pm

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. :)

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

Title: Re: [Math: Vectors] Clear my doubts please
Post by: Dasith on September 05, 2011, 05:58:13 pm

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. :)

& yeap i think i 4got da denominator , its P +( (-p+q)/3)= OX, but i dnt see anything wrong wid my calculations , can u tell me where exactly i went wrong in da calculations ?

Title: Re: [Math: Vectors] Clear my doubts please
Post by: Becca on September 18, 2011, 05:59:00 am

Hi Dasith, sorry for the late reply... >< I was a little tied up with my exams...
I shall include my working for the second part of the question here, the one on finding OX:

OX=OP+PX
    =p + 1/3(-p+q)
     =p - 1/3p+ 1/3q
     =2/3p + 1/3q

When I saw that Ricky said that the answer is '1/3(2q+p)' , I was stumped as it was different from the answer I got above. However, I checked the answer scheme, which said that my answer is correct. It's actually '1/3(2p+q)... Do you see any difference between your working and mine? :) Do let me know if you have any further difficulties...

Title: Re: [Math: Vectors] Clear my doubts please
Post by: Dasith on September 20, 2011, 04:43:04 pm

Quote from: Becca on September 18, 2011, 05:59:00 am

Hi Dasith, sorry for the late reply... >< I was a little tied up with my exams...
I shall include my working for the second part of the question here, the one on finding OX:

OX=OP+PX
    =p + 1/3(-p+q)
     =p - 1/3p+ 1/3q
     =2/3p + 1/3q

When I saw that Ricky said that the answer is '1/3(2q+p)' , I was stumped as it was different from the answer I got above. However, I checked the answer scheme, which said that my answer is correct. It's actually '1/3(2p+q)... Do you see any difference between your working and mine? :) Do let me know if you have any further difficulties...

isnt 2/3P + 1/3q the same as " P +( (-p+q)/3)"

simplify  it & u get da same O.o ! 

Title: Re: [Math: Vectors] Clear my doubts please
Post by: Becca on September 21, 2011, 03:27:55 pm

Hmm, are you trying to say that my method is quite long-winded??  :-[ Heheh, I get that a lot... I guess that the A level course that I am currently taking is getting to my head! XD  8)

Anyway, you were right about the answer to b(i) posted by Ricky being wrong... I missed that earlier. And yes, your method also gets the same answer of 2/3p+1/3q... but isn't it almost the same as my method? Hmm... I mean that isn't 'p + 1/3(-p+q)' the same as p +(-p+q)/3  :o

Other than the part where you forgot the denominator of 3 :) I think that you got the rest of the working correct... It all seems fine to me, though that little mistake did prevent you from getting the answer.

Ermm, do you understand what I'm trying to say?  :-\

Title: Re: [Math: Vectors] Clear my doubts please
Post by: Dasith on September 21, 2011, 05:38:02 pm

Quote from: Becca on September 21, 2011, 03:27:55 pm

Hmm, are you trying to say that my method is quite long-winded??  :-[ Heheh, I get that a lot... I guess that the A level course that I am currently taking is getting to my head! XD  8)

all wat i'm tryin 2 sat i missed da denominator 3 in my first post accidently , u r meathod is fine as well as mine, dosent make any of our ans wrong..

Quote from: Becca on September 21, 2011, 03:27:55 pm

Anyway, you were right about the answer to b(i) posted by Ricky being wrong... I missed that earlier. And yes, your method also gets the same answer of 2/3p+1/3q... but isn't it almost the same as my method? Hmm... I mean that isn't 'p + 1/3(-p+q)' the same as p +(-p+q)/3  :o

this is what i said in my previous post , when u simplify my ans u get yours :)

Quote from: Becca on September 21, 2011, 03:27:55 pm

Ermm, do you understand what I'm trying to say?  :-\

yeap , i think i misread some o u r posts O.o

Title: Re: [Math: Vectors] Clear my doubts please
Post by: Becca on September 22, 2011, 02:11:18 pm

 :D Oh, okay, that's settled then, I guess! It was quite an interesting question, I suppose... I hope that Ricky managed to read this in time for any exams he might have.  ;D

Title: Re: [Math: Vectors] Clear my doubts please
Post by: Dasith on September 22, 2011, 02:29:28 pm

Quote from: Becca on September 22, 2011, 02:11:18 pm

:D Oh, okay, that's settled then, I guess! It was quite an interesting question, I suppose... I hope that Ricky managed to read this in time for any exams he might have.  ;D

well checking on the dates i doubt it : ::) :-X

Title: Re: [Math: Vectors] Clear my doubts please
Post by: Becca on September 22, 2011, 02:41:43 pm

Oh, dear me... It is quite late.  :o