Explanation:
Let use the first of the medians. Medians AX.
Median AX is the midpoint of BC so our median is
Let use the second median.
Median BY is the midpoint of AC so our median is
Let use the final median.
Median CZ is the midpoint of AB so our median is
Now add all them up
Add like terms.
So
AX+BY+CZ=0