- #1
danago
Gold Member
- 1,123
- 4
Hey. I've got a vectors test coming up soon, so I've been going through everything. At the moment I am working on vector proofs. here's the question:
OABC is a parallelogram where [itex]\overrightarrow{OA}=\mathbf{a}[/itex] and [itex]\overrightarrow{OC}=\mathbf{c}[/itex]. M is the midpoint of the diagonal OB. Prove that the diagonals of this parallelogram bisect each other.
I drew a diagram
http://img102.imageshack.us/img102/7223/vectorproof0rg.gif
I already know that M is the midpoint of OB, so i just need to show that its also the midpoint of AC. This means that [itex]\overrightarrow{AM}=\overrightarrow{MC}[/itex], which is what I am proving.
[tex]
\overrightarrow{OB}=\overrightarrow{OC}+\overrightarrow{CB}
[/tex]
[tex]
\overrightarrow{OB}=\mathbf{c}+\mathbf{a}
[/tex]
[tex]
\overrightarrow{AM}=\overrightarrow{AO}+\overrightarrow{OM}
[/tex]
[tex]
\overrightarrow{AM}=\mathbf{-a}+\frac{1}{2}\overrightarrow{OB}
[/tex]
[tex]
\overrightarrow{AM}=\mathbf{-a}+\frac{1}{2}(\mathbf{c+a})
[/tex]
[tex]
\overrightarrow{AM}=\frac{1}{2}(\mathbf{c-a})
[/tex]
So now i know [itex]\overrightarrow{AM}[/itex] in terms of [itex]\mathbf{a}[/itex] and [itex]\mathbf{b}[/itex].
[tex]
\overrightarrow{MC}=\overrightarrow{MO}+\overrightarrow{OC}
[/tex]
[tex]
\overrightarrow{MC}=-\frac{1}{2}\overrightarrow{OB}+\mathbf{c}
[/tex]
[tex]
\overrightarrow{MC}=-\frac{1}{2}(\mathbf{c+a})+\mathbf{c}
[/tex]
[tex]
\overrightarrow{MC}=\frac{1}{2}(\mathbf{c-a})
[/tex]
So i now also have [itex]\overrightarrow{MC}[/itex]. Since they equal the same, as required, i can conclude that:
[tex]\overrightarrow{AM}=\overrightarrow{MC}[/tex]
Is that all i would need to do to answer the question? is the information i provided sufficient to prove that the two diagonals bisect each other?
Thanks,
Dan.
OABC is a parallelogram where [itex]\overrightarrow{OA}=\mathbf{a}[/itex] and [itex]\overrightarrow{OC}=\mathbf{c}[/itex]. M is the midpoint of the diagonal OB. Prove that the diagonals of this parallelogram bisect each other.
I drew a diagram
http://img102.imageshack.us/img102/7223/vectorproof0rg.gif
I already know that M is the midpoint of OB, so i just need to show that its also the midpoint of AC. This means that [itex]\overrightarrow{AM}=\overrightarrow{MC}[/itex], which is what I am proving.
[tex]
\overrightarrow{OB}=\overrightarrow{OC}+\overrightarrow{CB}
[/tex]
[tex]
\overrightarrow{OB}=\mathbf{c}+\mathbf{a}
[/tex]
[tex]
\overrightarrow{AM}=\overrightarrow{AO}+\overrightarrow{OM}
[/tex]
[tex]
\overrightarrow{AM}=\mathbf{-a}+\frac{1}{2}\overrightarrow{OB}
[/tex]
[tex]
\overrightarrow{AM}=\mathbf{-a}+\frac{1}{2}(\mathbf{c+a})
[/tex]
[tex]
\overrightarrow{AM}=\frac{1}{2}(\mathbf{c-a})
[/tex]
So now i know [itex]\overrightarrow{AM}[/itex] in terms of [itex]\mathbf{a}[/itex] and [itex]\mathbf{b}[/itex].
[tex]
\overrightarrow{MC}=\overrightarrow{MO}+\overrightarrow{OC}
[/tex]
[tex]
\overrightarrow{MC}=-\frac{1}{2}\overrightarrow{OB}+\mathbf{c}
[/tex]
[tex]
\overrightarrow{MC}=-\frac{1}{2}(\mathbf{c+a})+\mathbf{c}
[/tex]
[tex]
\overrightarrow{MC}=\frac{1}{2}(\mathbf{c-a})
[/tex]
So i now also have [itex]\overrightarrow{MC}[/itex]. Since they equal the same, as required, i can conclude that:
[tex]\overrightarrow{AM}=\overrightarrow{MC}[/tex]
Is that all i would need to do to answer the question? is the information i provided sufficient to prove that the two diagonals bisect each other?
Thanks,
Dan.
Last edited by a moderator: