# vector problem

• Sep 16th 2009, 03:16 AM
smmmc
vector problem
Hey

OABCDEFG is a cuboid

-> -> ->
OA= 3j OC=2k OD=i

M is such that ->
OM= 1/3 ->
OE

N is the midpoint of BF. Find
a) ->
MN

not sure how to start, i drew a cuboid but dont know how to find the 1/3 of OE

thanks
Thanks
• Sep 16th 2009, 05:52 AM
Hello smmmc
Quote:

Originally Posted by smmmc
Hey

OABCDEFG is a cuboid

-> -> ->
OA= 3j OC=2k OD=i

M is such that ->
OM= 1/3 ->
OE

N is the midpoint of BF. Find
a) ->
MN

not sure how to start, i drew a cuboid but dont know how to find the 1/3 of OE

thanks
Thanks

I'm assuming that the vertices are labelled as in the attached diagram, so that, for instance, BF is parallel to CG, OD and AE.

If this is the case, then $\vec{BF}=\vec{OD} = \vec{i}$ and $\vec{CB}=\vec{OA}=3\vec{j}$

$\Rightarrow \vec{BN}=\tfrac12\vec{BF}=\tfrac12\vec{i}$

$\Rightarrow \vec{ON} = \vec{OC}+\vec{CB}+\vec{BN}$

$= 2\vec{k}+3\vec{j}+\tfrac12\vec{i}$

Also $\vec{OE} = \vec{OD}+\vec{DE} = \vec{i}+3\vec{j}$

$\Rightarrow \vec{OM} = \tfrac13\vec{OE}=\tfrac13\vec{i} + \vec{j}$

Now $\vec{ON}=\vec{OM}+\vec{MN}$

$\Rightarrow \vec{MN} = \vec{ON}-\vec{OM}$

$= 2\vec{k}+3\vec{j}+\tfrac12\vec{i}-\tfrac13\vec{i} - \vec{j}$

$=\tfrac16\vec{i}+2\vec{j}+2\vec{k}$