Proof using vectors: Sum of medians of triangle equal to zero - http://ocw.mit.edu/courses/mathematics/18-02sc-multivariable-calculus-fall-2010/1.-vectors-and-matrices/part-a-vectors-determinants-and-planes/session-1-vectors/MIT18_02SC_we_4_comb.pdf

Using A as the origin and including A so that AB =A+B
The above statement is true. However what did they take as the origin in the document? As I tried an arbitrary origin but could not get what they obtained.

thanks

Question

Proof using vectors: Sum of medians of triangle equal to zero - http://ocw.mit.edu/courses/mathematics/18-02sc-multivariable-calculus-fall-2010/1.-vectors-and-matrices/part-a-vectors-determinants-and-planes/session-1-vectors/MIT18_02SC_we_4_comb.pdf

Using A  as the origin and including A so that AB =A+B
The above statement is true. However what did they take as the origin in the document? As  I tried an arbitrary origin but could not get what they obtained.

thanks