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.
In ∆ABC,
∠BAC + ∠ABC + ∠ACB = 180°
| Sum of all the angles of a triangle is 180°
⇒ ∠BAC + 69° + 31° = 180°
⇒ ∠BAC + 100°= 180°
⇒ ∠BAC = 180° - 100° = 80° ...(1)
Now, ∠BDC = ∠BAC
| Angles in the same segment of a circle are equal
= 80°. | Using (1)
If the non-parallel sides of a trapezium are equal, prove that it is cyclic. Prove that an isosceles trapezium is cyclic.