Originally Posted by
Moo If two vectors are parallel, there exists a real number x, different to 0, such that :
$\displaystyle \vec{u}=x \vec{v}$
So seeing that $\displaystyle 4={\color{red}\frac 23} \cdot 6$ and $\displaystyle 6={\color{red}\frac 23} \cdot 9$, can you set up a relation between -2 and m ?
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Now, if two vectors are perpendicular, this means that the dot product is 0.
For two vectors : $\displaystyle u=\begin{pmatrix} u_1 \\ u_2 \\ u_3 \end{pmatrix}$ and $\displaystyle v=\begin{pmatrix} v_1 \\ v_2 \\ v_3 \end{pmatrix}$, their dot product is :
$\displaystyle u_1v_1+u_2v_2+u_3v_3$
So here, what is the dot product of u and v ? Deduce the value of m ?