Theorems on Cyclic Quadrilateral
In this section we will discuss theorems on cyclic quadrilateral.A quadrilateral is called Cyclic quadrilateral if its all vertices lie on the circle.
It has some special properties which other quadrilaterals, in general, need not have. Here we have proved some theorems on cyclic quadrilateral.
1) The opposite angles of a Cyclic  quadrilateral are supplementary.
Given : A cyclic quadrilateral ABCD.
Prove that : ∠A + ∠C = 180° ang ∠B + ∠D = 180°
Construction : Join AC and BD.


1) ∠ACB = ∠ADB  1) Angles in the same segment. 
2) ∠BAC = ∠BDC  2) Angles in the same segment 
3)∠ACB + ∠BAC = ∠ADB + ∠BDC  3) Addition property 
4) ∠ACB + ∠BAC = ∠ADC  4) Add ∠ABC on both sides. 
5) ∠ABC + ∠ACB + ∠BAC = ∠ABC + ∠ADC  5) From Above. 
6) 180^{o} = ∠ABC + ∠ADC  6) Sum of the angle of a triangle is 180^{o} 
7) ∠B + ∠D = 180^{o}  7) Opposite angles of cyclic quadrilateral. 
8) ∠A + ∠B + ∠C + ∠D  8) Measure of a quadrilateral. 
9) ∠A + ∠C = 360^{o}  (∠B + ∠D)  9) From Above. 
10) ∠A + ∠C = 360^{o}  180^{o} = 180^{o}  10) Angle sum property 
11) ∠A + ∠C = 180^{o} and ∠B + ∠D = 180^{o}  11) From above .So opposite angles are supplementary. 
Given : A cyclic quadrilateral ABCD one of whose side AB is produced to E.
Prove that : ∠CBE = ∠ADC


1) ∠ABC + ∠ADC = 180^{o}  1) Opposite angles of cyclic quadrilateral 
2) ∠ABC + ∠CBE = 180^{o}  2) Linear Pair angles. 
3) ∠ABC + ∠ADC = ∠ABC + ∠CBE  3) From above. 
4) ∠ADC = ∠CBE  4) subtraction property 
Theorems on Circle
• Theorems on Chord
• Theorems on Chord and Subtended Angle
• Theorems on Arc and Angle
• Theorems on Cyclic Quadrilateral