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Theorems on Angles in a Circle

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CISCE: Class 10

Theorem: Angle at the Centre

Statement:
The angle subtended by an arc at the centre of a circle is double the angle subtended by it at any point on the remaining part of the circle.

Result:

∠AOB = 2∠ACB

Short Proof (Idea):

  • Join the centre to the points on the circle.

  • Radii form isosceles triangles.

  • Angle at centre = sum of angles at the circle.

  • Hence, the angle at the centre is double the angle at the circle.

CISCE: Class 10

Theorem: Angles in the Same Segment

Statement:
Angles in the same segment of a circle are equal.

Result:

∠ACB = ∠ADB

Short Proof (Idea):

  • Both angles stand on the same arc.

  • The angle at the centre is double each of them.

  • Hence, both angles are equal.

CISCE: Class 10

Theorem: Angle in a Semi-circle

Statement:
The angle in a semicircle is a right angle.

Result:

∠ACB = 90

Short Proof (Idea):

  • Diameter subtends an angle of 180° at the centre.

  • The angle at the circle is half of it.

  • Therefore, angle = 90°.

CISCE: Class 10

Theorem: Converse: Angle in a Circle

Statement:
If an arc of a circle subtends a right angle at any point on the remaining part of the circle, then the arc is a semicircle.

Result:
Arc AB is a semicircle
(or AB is a diameter)

Short Proof (Idea):

  • Given angle at the circle ∠ACB = 90°.

  • The angle at the centre is double the angle at the circle.

  • Therefore, ∠AOB = 2 × 90° = 180°.

  • Hence, A, O, and B lie on a straight line, so AB is a diameter.

  • Therefore, arc AB is a semicircle.

Video Tutorials

We have provided more than 1 series of video tutorials for some topics to help you get a better understanding of the topic.

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Shaalaa.com | Circles (Arc Properties Part 3)

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