Vectors in a plane I have a question where i need to prove that three vectors are all on one plane.
I proved that the angle between ALL the three vectors is 120. 
Is that enough to prove that it is on one plane? because together they are 360.
Basiclly, is there any other way to build three vectors with 120 angle between them, without being all on one plane
Thanks!

Using spherical trigonometry, there is one triangle with sides of $\frac{2\pi}3$. Using the Law of Cosines for Spherical Triangles, we can compute each angle of the triangle to be
$$
\begin{align}
\cos(\theta)
&=\frac{\cos(\frac{2\pi}3)-\cos(\frac{2\pi}3)\cos(\frac{2\pi}3)}{\sin(\frac{2\pi}3)\sin(\frac{2\pi}3)}\\
&=\frac{-\frac12-(-\frac12)(-\frac12)}{\frac{\sqrt3}2\cdot\frac{\sqrt3}2}\\
&=-1
\end{align}
$$
That is, each angle of the triangle is $\pi$. Thus, the triangle forms a great circle, which is the intersection of a plane through the center of the sphere with the sphere.
This means that the three vectors are coplanar.
