- #1

Grand

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## Homework Statement

Let [tex]z_1[/tex], [tex]z_2[/tex], [tex]z_3[/tex] and [tex]z_4[/tex] be te position vectors of the vertices of quadrilateral AMCD. Prove that ABCD is a parallelogram if and only if [tex]z_1-z_2-z_3+z_4=0[/tex].

## Homework Equations

## The Attempt at a Solution

The solution obviously uses the fact that collinear vectors of equal magnitude are equal, but I get [tex]z_1-z_2+z_3-z_4=0[/tex]. Am I missing something obvious or is it just notations issue with the vertices of the quadrilateral.