Given: ABCD is a rectangle.
Join AC and BD.
Now, In △ABC,
AC2=AB2+BC2 (I) (By Pythagoras theorem)
Similarly, In △ABD,
BD2=AD2+AB2 (II) (By Pythagoras theorem)
Similarly, In △BDC,
BD2=BC2+CD2 (III) (By Pythagoras theorem)
Similarly, In △ACD,
AC2=AD2+CD2 (IV) (By Pythagoras theorem)
Adding I, II, III, IV
2(AC2+BD2)=2(AB2+BC2+CD2+AD2)
AC2+BD2=AB2+BC2+CD2+AD2