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quadrilateral inscribed in a circle

Quadrilateral Inscribed in a Circle

In the following blog we will learn about the quadrilateral inscribed in a circle. quadrilateral inscribed in a circle kind of question are common in CAT examinations.



An inscribed quadrilateral is any four sided figure whose vertices all lie on a circle. (The sides are therefore chords in the circle!) This conjecture give a relation between the opposite angles of such a quadrilateral. It says that these opposite angles are in fact supplements for each other. In other words, the sum of their measures is 180 degrees.

For the quadrilateral shown above, this conjecture says that:

AngleA + AngleC = 180AngleB + AngleD = 180

The precise statement of the conjecture is:

Conjecture (Quadrilateral Sum ): Opposite angles in any quadrilateral inscribed in a circle are supplements of each other. (Their measures add up to 180 degrees.)

Proof: We use ideas from the Inscribed Angles Conjecture to see why this conjecture is true. The main result we need is that an inscribed angle has half the measure of the intercepted arc. Here, the intercepted arc for Angle(A) is the red Arc(BCD) and for Angle(C) is the blue Arc(DAB).

Arc(BCD)+Arc(DAB) = 360

2*Angle(A)+2*Angle(C) = 2*[Angle(A)+Angle(C)] = 360

Angle(A)+Angle(C) = 180The proof is complete!

Arc(BCD)=2*Angle(A) and Arc(DAB)=2*Angle(C)

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