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India
Class IX

Cyclic Quadrilaterals

Lesson

A cyclic quadrilateral is a four-sided shape that has all its vertices touching the circle's circumference, such as the one shown below.

 

The Theorem

The opposite angles in a cyclic quadrilateral add up to $180^\circ$180°.

 

Proof:

 

  $ABCD$ABCD is a cyclic quadrilateral (given)
Join $AC$AC to $BD$BD  
$\angle CAB+\angle ABC+\angle ACB$CAB+ABC+ACB $=$= $180^\circ$180° (angle sum of a triangle)
$\angle CAB$CAB $=$= $\angle CDB$CDB (angles in the same segment of a circle are equal)
$\angle ACB$ACB $=$= $\angle ADB$ADB (angles in the same segment of a circle are equal)

Therefore, adding the previous two statements we get

$\angle ACB+\angle CAB=\angle ADB+\angle CDB$ACB+CAB=ADB+CDB $=$= $\angle ADC$ADC  
$\angle ACB+\angle CAB+\angle ABC$ACB+CAB+ABC $=$= $180^\circ$180° then - Adding $\angle ABC$ABC on both sides
$\angle ACB+\angle CAB+\angle ABC$ACB+CAB+ABC $=$= $180^\circ$180° (Angle sum of a triangle)
$\angle ADC+\angle ABC$ADC+ABC $=$= $180^\circ$180°  
$\angle BAD+\angle BCD=360^\circ-\left(\angle ADC+\angle ABC\right)$BAD+BCD=360°(ADC+ABC) $=$= $180^\circ$180°  

 

Remember!

The converse of this theorem is also true.

If a pair of opposite angles of a quadrilateral is supplementary, then the quadrilateral is cyclic.

 

Worked Examples

Question 1

In the diagram, $O$O is the centre of the circle. Show that $x$x and $y$y are supplementary angles.

Question 2

Consider the figure:

  1. Prove that $\angle ABC$ABC = $\angle CDE$CDE.

  2. By proving two similar triangles, Prove that $\angle BAD$BAD and $\angle DCE$DCE are equal.

  3. Using this prove that $EB\times EC=ED\times EA$EB×EC=ED×EA.

 

 

 

 

 

 

Outcomes

9.G.C.3

The angle subtended by an arc at the centre is double the angle subtended by it at any point on the remaining part of the circle. Angles in the same segment of a circle are equal. if a line segment joining two points subtends equal angle at two other points lying on the same side of the line containing the segment, the four points lie on a circle. The sum of the either pair of the opposite angles of a cyclic quadrilateral is 180 degree and its converse.

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