NCERT Solutions
Class 9 Maths
Quadrilaterals

Ex.8.1 Q.4
Show that the diagonals of a square are equal and bisect each other at right angles.
Let ABCD be a square. Let the diagonals AC and BD intersect each other at a point O.
To prove that the diagonals of a square are equal and bisect each other at right angles, we have to
prove AC = BD, OA = OC, OB = OD, and ∠AOB = 900.
In ΔABC and ΔDCB,
AB = DC
(Sides of a square are equal to each other)
∠ABC = ∠DCB
(All interior angles are of 900)
BC = CB
(Common side)
So, ΔABC ≅ ΔDCB
(By SAS congruency)
Hence, AC = DB
(By CPCT)
Hence, the diagonals of a square are equal in length.
In ΔAOB and ΔCOD,
∠AOB = ∠COD
(Vertically opposite angles)
∠ABO = ∠CDO
(Alternate interior angles)
AB = CD
(Sides of a square are always equal)
So, ΔAOB ≅ ΔCOD
(By AAS congruence rule)
Hence, AO = CO and OB = OD
(By CPCT)
Hence, the diagonals of a square bisect each other.
In ΔAOB and ΔCOB,
As we had proved that diagonals bisect each other, therefore,
AO = CO
AB = CB
(Sides of a square are equal)
BO = BO
(Common)
So, ΔAOB ≅ ΔCOB
(By SSS congruency)
Hence, ∠AOB = ∠COB
(By CPCT)
However, ∠AOB + ∠COB = 1800
(Linear pair)
2∠AOB = 1800
∠AOB = 900
Hence, the diagonals of a square bisect each other at right angles.