REF.IMAGE.
Given: ΔABC and O is a point in thw inteeior of a $ \Delta ABC,
whereOD⊥BC,OE⊥AC,OF⊥AB.
proof:-
Join the point O from A,B and C.
Using pythagoras theorem
In rt.angled ΔleOAF,OA2=OF2+AF2...(1)
In rt. angled ΔleODB,.OB2=OD2+BD2...(2)
In rt. angled ΔleOEC,.OC2=OE2+EC2...(3)
Adding (1),(2) & (3)
OA2+OB2+OC2=AF2+OF2+OD2+BD2+OE2+EC2
OA2+OB2+OC2=OD2+OE2+OF2+AF2+BD2+CE2
Hence proved