Question 675575
{{{drawing( 600,600, -5, 15, -5, 15, 
         grid(0),locate( 0.3, 0, A),locate( 0.3, 10, D ),locate( 10, 10, C),locate( 10.3, 0, B ),locate( 5.3, 5.3, O ),
         line( 0, 10, 10, 0 ),line( 10, 10, 10, 0 ),line(0, 10, 10, 10 ),line( 0, 0, 10, 10 )
)}}}


< {{{DAB=90}}}

so, <{{{ DAO}}}=< {{{BAO}}}={{{45}}}

=> {{{DO=AO}}} .........(Sides opposite equal angles are equal)

Similarly {{{AO=OB=OC}}} can be proved

This gives the proof of diagonals of square being equal.