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    Hi,
    We started vectors and to be honest, i find it quite baffling when others are getting on well. Anyways, i was doin some q's and wasnt sure about this one :



    Points A and B have coordinates (2,7) and (-3,-3- respectively. Use a vector method to find the coordinates of C and D, where

    a) C is the point such that \vec{AC}=    3\vec{AB}

    Working : -


     \vec{AB}  =  -\vec{OA}        +   \vec{OB}

    = (-3 , -3) (in vector form) - (2,7) (in vector form)

    hence

     \vec{AB} = (-5, -10 )


     \vec{AC}  = 3 \vec{AB} = 3 . (-5, - 10) = (-15, -30)


    However, this is not the right answer!


    THanks
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    You've worked out the vector AC, you've been asked for the point C. Your method seems right upto there
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    (Original post by DavyS)
    You've worked out the vector AC, you've been asked for the point C. Your method seems right upto there

    Do u mean that i have to work out what the vector OC is in order to get the point C ?
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    (Original post by wizz_kid)
    Do u mean that i have to work out what the vector OC is in order to get the point C ?
    :yep:
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    (Original post by yusufu)
    :yep:

    Oh i c!

    So OC = - AC - OA

    = - (-15, -30 ) - (2, 7)

    = (13 , 23 )

    Is that right?
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    (Original post by wizz_kid)
    Do u mean that i have to work out what the vector OC is in order to get the point C ?
    Yes, do you see why? (And you can easily get there if you have AC and replace the point A with the origin)

    edit: No, you've got the vector in the wrong direction, should be negative those... unless I've gone mad.
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    (Original post by nota bene)
    Yes, do you see why? (And you can easily get there if you have AC and replace the point A with the origin)
    :yep:
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    (Original post by DavyS)
    You've worked out the vector AC, you've been asked for the point C. Your method seems right upto there

    So OC = - AC - OA

    = - (-15, -30 ) - (2, 7)

    = (13 , 23 )

    Is that right?
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    (Original post by nota bene)
    Yes, do you see why? (And you can easily get there if you have AC and replace the point A with the origin)

    edit: No, you've got the vector in the wrong direction, should be negative those... unless I've gone mad.
    y should they be negative?
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    Because OC=OA+AC
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    (Original post by nota bene)
    Because OC=OA+AC
    oooooooyea!

    THanks


    If u dont mind, could u guide me through this q as wel plz :



    C is the point on AB such that AC : CB = 4 : 3. Express c in terms of a and b.

    Thanks!
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    AB=?

    Note that AC=(4/7)AB and/or CB=(3/7)AB
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    (Original post by nota bene)
    AB=?

    Note that AC=(4/7)AB and/or CB=(3/7)AB

    Yea.

    How can u find AB from that?
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    (Original post by wizz_kid)
    Yea.

    How can u find AB from that?
    You can't. Or rather, that hint had nothing to do with finding AB, finding AB is quite trivial and you should be able to do it yourself, as you have done very similar stuff earlier in the thread. Draw a sketch if you are unsure.
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    Wiki Support Team
    I've answered this in your other thread.
 
 
 
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