Prove that the diagonals of a rectangle are of equal length.

ABCD is a rectangle with AC and BD as its diagonals.

ABCD is a rectangle

∴∠A = 90^{o}, AD = BC

AD//BC and AB is a transversal

∴∠A +∠B = 180^{o}

⇒ ∠B = 90^{o}

In ∆ ABD and ∆ BAC

AB = BA

∠A = ∠B

AD = BC

∴∆ABD ∆BAC

⇒ BD = AC (c.p.c.t)

Hence, the diagonals of a rectangle are equal.