# Thread: Proving Vectors are collinear under certain conditions

1. ## Proving Vectors are collinear under certain conditions

Three points X, Y, Z have position vectors x,y,z. Show that X, Y and Z are collinear iff x ^ y + y ^ z + z ^ x = 0.

Here "^" denotes the vector cross product

2. Originally Posted by Paul616
Three points X, Y, Z have position vectors x,y,z. Show that X, Y and Z are collinear iff x ^ y + y ^ z + z ^ x = 0.

Here "^" denotes the vector cross product
If X, Y and Z are collinear then $z=\lambda x + (1-\lambda)y$ for some scalar $\lambda$. You can then verify that $y\times z + z\times x + x\times y=0$ (remembering that the cross product of a vector with itself is always 0).

For the converse, if $y\times z + z\times x + x\times y=0$ then $0 = x.(y\times z) + x.(z\times x) + x.(x\times y) = x.(y\times z)$ (since the other two terms are 0). So $y\times z$ is orthogonal to x. But it is also orthogonal to y and z. If x, y and z are linearly independent then $y\times z$ is orthogonal to the whole space and is therefore 0. But that would mean that y and z are not linearly independent. That contradiction shows that the three vectors must be linearly dependent.

So one of them, z say, is a linear combination of the others, $z = \lambda x + \mu y$. Substitute that value for z into the equation $y\times z + z\times x + x\times y=0$ and you will find that $\mu = 1-\lambda$, which is the condition for X, Y and Z to be collinear.

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# vectors conditions to be collinear

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