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I am trying to show that vectors $a= i + 2j +3k$, $b = 4i + 5j +6k$ , $c= 7i +8j+9k$ all lie in the same plane.

I have looked up online and saw that to show that, I have to show that -

$a\cdot (b \times c) = 0$ , using both the cross and dot product.

I am not sure in the understanding behind why I need to prove that to show that all 3 vectors lie in the same plane.

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  • $\begingroup$ $2b=a+c{}{}{}{}$. $\endgroup$ – Angina Seng Feb 15 '18 at 7:25
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Yes triple product is a correct metod to verify whether or not three vectors lie in the same plane, indeed

  • cross product $\vec b\times \vec c\,$ gives a vector normal to the plane and
  • $\vec a\cdot (b\times c)=0\,$ gives the conditon that also $\vec a$ is in the plane of $\vec b$ and $\vec c$

More in general note that the triple product $\vec a\cdot (\vec b\times \vec c)$ is the (signed) volume of the parallelepiped defined by the three vectors given, thus it is equal to zero if and only if the three vectors lie on the same plane.

enter image description here

As an alternative we can also verify whether or not the three vectors are linearly dependent by inspection or by the standard method of matrix RREF.

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  • $\begingroup$ But if it’s equals to 0. the theta of the dot product is 9@ degrees. Meaning it’s perpendicular. How does showing that they are perpendicular mean that they are on the same plane ? Sorry I have difficulty visualising this.. $\endgroup$ – user175089 Feb 15 '18 at 7:55
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    $\begingroup$ The key point is that $n= b \times c$ is a normal vector to the plane b-c thus if $a\cdot n=0$ also $a$ is in the same plane. $\endgroup$ – user Feb 15 '18 at 7:58

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