AB^2=AH^2 + HB^2
AC^2=AH^2 + CH^2=> AH^2=AC^2-CH^2
AB^2=AC^2 - CH^2 + HB^2
AB^2=BC^2 - (BC-HB) ^2 + BH^2
AB^2=BC^2 - BC^2 + 2*BC*BH-BH^2+BH^2=2*BC*BH
както так
