In figure A,B and C are three points on a circle with centre O such that ∠ BOC = 30° and ∠ AOB = 60°. If D is a point on the circle other than the arc ABC, find ∠ADC.
∠CDB = ∠BAC
| Angles in the same segment of a circle are equal
= 30° ...(1)
∠DBC = 70° ...(2)
In ∆BCD,
∠BCD + ∠DBC + ∠CDB = 180°
| Sum of all the angles of a triangle is 180°
⇒ ∠BCD + 70° + 30° = 180°
| Using (1) and (2)
⇒ ∠BCD + 100° = 180°
⇒ ∠BCD = 180° - 100°
⇒ ∠BCD = 80° ...(3)
In ∆ABC,
AB = BC
∴ ∠BCA = ∠BAC
| Angles opposite to equal sides of a triangle are equal
= 30° ...(4)
| ∵ ∠BAC = 30° (given)
Now, ∠BCD = 80° | From (3)
⇒ ∠BCA + ∠ECD = 80°
⇒ 30° + ∠ECD = 80°
⇒ ∠ECD = 80° - 30°
⇒ ∠ECD = 50°.
If the non-parallel sides of a trapezium are equal, prove that it is cyclic. Prove that an isosceles trapezium is cyclic.