# NCERT Class 9 Solutions: Quadrilaterals (Chapter 8) Exercise 8.1 – Part 2

Vertically opposite angles

Vertically Opposite Angles are the angles opposite each other when two lines cross. "Vertical" in this case means they share the same Vertex (or corner point), not the usual meaning of up-down.

Q-3 Show that if the diagonals of a quadrilateral bisect each other at right angles, then it is a rhombus..

• Let ABCD be a quadrilateral whose diagonals bisect each other at right angles.Given,

• To show,

ABCD is parallelogram and

• Proof,

In and ,

(Given)

(Each , Opposite sides of a parallelogram are equal)

OB=OB (Common)

Therefore, by Side-Angle-Side congruence condition.

• Thus , by Corresponding Parts of Congruent Triangles

• Similarly we can prove,

• Opposites sides of a quadrilateral are equal hence ABCD is a parallelogram.Thus, ABCD is rhombus as it is a parallelogram whose diagonals intersect at right angle.

Q-4 Show that the diagonals of a square are equal and bisect each other at right angles.

Solution:

• Let ABCD be a square and its diagonals AC and BD intersect each other at O.To show,

,

• Proof,In , (Common) (Given)

• Therefore, by Side-Angle-Side congruence condition.

• Thus, by Corresponding Parts of Congruent Triangles. Therefore, diagonals are equal.

• Now,In (Alternate interior angles) (Vertically opposite triangle) (Given)

• Therefore by Angle-Angle-Side congruence condition.

• Thus, by Corresponding Parts of Congruent Triangles (Diagonal bisect each other.)

• Now,, (Given) (diagonals are bisected)(Sides of the square)

• Therefore, by SSS congruence condition.also, (Linear pair)

• Thus, (Diagonals bisect each other at right angles)

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