Geometry Problem 1390: Triangle, any point, perpendicular, sum of the squares, Tutoring

Proposition

The figure below shows a triangle ABC and a point P. PD, PE and PF are perpendicular to AB, BC, and AC, respectively. Prove that AD2 + BE2 + CF2 = BD2 + EC2 + FA2.
 

Geometry Problem 1390: Triangle, any point, perpendicular, sum of the squares, Tutoring