Arc and Angles
Arc and angles Subtended by them
Some results on angles subtended by arcs of a circle either at the center of the circle or at a point on its circumference.
1) The angle subtended by an arc of a circle at the center is double the angle subtended by it at any point on the remaining part of the circle.
∠POQ = 2 ∠PRQ or ∠ PRQ = ½ ∠ POQ
2) Angles in the same segment of a circle are equal.
∠PRQ = ∠PSQ ( both the angles are subtended in the same arc PQ )
3) The angle in a semicircle is a right angle.
∠PRQ = 900 (PQ is a diameter )
Some solved examples on arc and angles
1) In the given figure, A,B,C and D are four points on a circle. AC and BD intersect at a point E such that ∠BEC = 1300 and ∠ECD = 200. Find ∠BAC.
Solution : ∠BEC = 1300 and ∠ECD = 200
∠BEC + ∠CED = 1800 ( Linear pair angles)
130 + ∠CED = 180
∴ ∠CED = 500
∠EDC = 180 – (50 +20) [ In ΔCDE, sum of all the angles in a Δ = 1800 )
∠EDC = 1100
But ∠EDC = ∠CDB ( angles in the same segment are equal )
∴ ∠CDB = 1100
2) Two circles are drawn with sides AB and AC of a triangle ABC as diameters. The circles intersect at a point D. Prove that D lies on BC.
Solution : Join AD.
As AC is a diameter of circle C1
∴ ∠ADC = 900 [ Angle in a semicircle is a right angle (90) ]
As AB is a diameter of circle C2
∴ ∠ADB = 900 [ Angle in a semicircle is a right angle (90) ]
∠ ADC + ∠ADB = 90 + 90 = 1800
⇒ BDC is a straight line ⇒ D lies on BC.
Circles Home Page
• Parts of Circle
• Arc and Chords
• Equal Chords of a Circle
• Arc and Angles
• Cyclic Quadrilaterals
• Tangent to Circle