Question

Prove the opposite side of the quadrilateral circumscribing a circle subtend supplementary angle at the centre of a circle

Answer 1

Let ABCD be a quadrilateral circumscribing a circle centered at O such that it touches the circle at point P, Q, R, S. Let us join the vertices of the quadrilateral ABCD to the center of the circle.

In ΔOAP and ΔOAS,

AP = AS (Tangents from the same point)

OP = OS (Radii of the same circle)

OA = OA (Common side)

ΔOAP ≅ ΔOAS (SSS congruence criterion)

∠POA = ∠AOS  (CPCT)

or ∠1 = ∠8

Similarly,

∠2 = ∠3

∠4 = ∠5

∠6 = ∠7

∠1 + ∠2 + ∠3 + ∠4 + ∠5 + ∠6 + ∠7 + ∠8 = 360º

(∠1 + ∠8) + (∠2 + ∠3) + (∠4 + ∠5) + (∠6 + ∠7) = 360º

2∠1 + 2∠2 + 2∠5 + 2∠6 = 360º

2(∠1 + ∠2) + 2(∠5 + ∠6) = 360º

(∠1 + ∠2) + (∠5 + ∠6) = 180º

∠AOB + ∠COD = 180º

Similarly, we can prove that ∠BOC + ∠DOA = 180º

Hence, opposite sides of a quadrilateral circumscribing a circle subtend supplementary angles at the centre of the circle.

Answer 2

To Prove : ∠AOB + ∠COD = 180°;∠BOC + ∠DOA = 180°

Given: ABCD is circumscribing the circle.

Proof:

Let ABCD be a quadrilateral circumscribing a circle centred at O such that it touches the circle at point P, Q, R, S.

join the vertices of the quadrilateral ABCD to the centre of the circle.

Consider ΔOAP and ΔOAS:

AP = AS (Tangents from the same point)

OP = OS (Radii of the same circle)

OA = OA (Common side)

ΔOAP ≅ ΔOAS (SSS congruence criterion)

And thus, ∠ POA = ∠ AOS

∠1 = ∠ 8 Similarly,

∠2 = ∠ 3

∠4 = ∠ 5

∠6 = ∠ 7

∠1 + ∠ 2 + ∠ 3 + ∠ 4 + ∠ 5 + ∠ 6 + ∠ 7 + ∠ 8 = 360°

(∠ 1 + ∠ 8) + (∠ 2 + ∠ 3) + (∠ 4 + ∠ 5) + (∠ 6 + ∠ 7) = 360°

2∠ 1 + 2∠ 2 + 2∠ 5 + 2∠ 6 = 360°

2(∠ 1 + ∠ 2) + 2(∠ 5 + ∠ 6) = 360°

(∠ 1 + ∠ 2) + (∠ 5 + ∠ 6) = 180°

∠ AOB + ∠ COD = 180°

Similarly, we can prove that ∠ BOC + ∠ DOA = 180°

Hence, opposite sides of a quadrilateral circumscribing a circle subtend supplementary angles at the centre of the circle.

[Proved] Q.E.D