D-------------------B-------------------E
\ + / \ + / \
\ + / \ + / \
\ + / \ + / \
\ + / \ + / \
\ G---------\---------H \
\ / + \ / + \
\ / + \ / + \
\ / + \ / + \
\ / + \ / + \
A----- -------------F-------------------C
1, BF is a median of triangle ABC and triangle DEF; ABC and DEF are congruent.
2. BF & GH are diagonals of parallelogram BHFG
3. BG = BA/2; BH = BC/2; GH = AF = AC/2.
4, (BF)^2 + (GH)^2 = 2*[(BG)^2 + (BH)^2} {Formula for diagonals of a parallelogram}
5, (BF)^2 = 2*[(BG)^2 + (BH)^2} - (GH)^2
6, (BF)^2 = 2*[(BA/2)^2 + (BC/2)^2} - (AC/2)^2