Ex.8.1 Q7 Quadrilaterals Solution - NCERT Maths Class 9

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\(ABCD\) is a rhombus. Show that diagonal \(AC\) bisects \(\angle A\) as well as \(\angle C\) and diagonal \(BD\) bisects \(\angle B\) as well as \(\angle D\).


 Video Solution
Ex 8.1 | Question 7

Text Solution


What is known?

ABCD is a rhombus.

What is unknown?

How we can show that diagonal \(AC\) bisects \(∠A\) as well as \(∠C\) and diagonal BD bisects \(∠B\) as well as \(∠D.\)


We can use alternate interior angles property to show diagonal \(AC\) bisects angles \(A\) and \(C\), similarly diagonal \(BD\) bisects angles B and D.


Let us join \(AC\).

In \(\Delta\)\(ABC\),

\[\begin{align}&BC=AB\\&\left( \begin{array}  & \text{Sides of a rhombus are} \\ \text{ equal to each other} \\ \end{array} \right)\\\\&\angle1=\angle2\\&\left( \begin{array}  & \text{Angles opposite to equal sides } \\   \text{ of a triangle are equal} \\ 
\end{array} \right)\end{align}\]

However, \(\angle 1 = \angle 3\) (Alternate interior angles for parallel lines \(AB\) and \(CD\))

\(\angle 2 = \angle 3\)

Therefore, \(AC\) bisects \(\angle C\).

Also, \(\angle 2 = \angle 4\) (Alternate interior angles for || lines \(BC\) and \(DA\)) \(\angle 1 = \angle 4\)

Therefore, \(AC\) bisects \(\angle A\).

Similarly, it can be proved that \(BD\) bisects \(\angle B\) and \(\angle D\) as well.

 Video Solution
Ex 8.1 | Question 7