Let ABCD is a rectangle

AC and BD are the diagonals of rectangle

In ΔABC and ΔBCD, we have

AB = CD (Opposite sides of rectangle are equal)

∠ABC = ∠BCD (Each equal to 90^o)

BC = BC (Common)

Therefore,

ABC BCD (By SAS congruence criterion)

AC = BD (c.p.c.t)

Hence, the diagonals of a rectangle are equal.