A point O in the interior of a rectangle ABCD is joined with each of the vertices A, B, C and D. Prove that: OA2 + OC2 = OB2 + OD2.
Through O, draw EOF || AB. Then, ABFE is a rectangle.
In right triangles OEA and OFC, we have:
OA2 = OE2 + AE2
OC2 = OF2 + CF2
∴OA2 + OC2 = OE2 + OF2 + AE2 + CF2 ---------(i)
Again, in right triangles OFB and OED, we have:
OB2 = OF2 + BF2
OD2 = OE2 + DE2
∴OB2 + OD2 = OF2 + OE2 + BF2 + DE2
=OE2 + OF2 + AE2 + CF2 --------------(ii) [∴ BF = AE & DE = CF]
From (i) and (ii), we get the result.