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 ∆OAB and ∆OCD,
OA = OC | Radii of a circle
OB = OD | Radii of a circle
∠AOB = ∠COD
| Vertically Opposite Angles
∴ ∆OAB ≅ ∆OCD     | SAS Rule
∴ AB = CD     | C.P.C.T.
⇒ Arc AB = Arc CD    ...(1)
Similarly, we can show that
Arc AD = Arc CB Â Â Â ...(2)
Adding (1) and (2), we get
Arc AB + Arc AD = Arc CD + Arc CB
⇒     Arc BAD = Arc BCD
⇒ BD divides the circle into two equal parts (each a semicircle)
∴ ∠A = 90°, ∠C = 90°
| Angle of a semi-circle is 90°
Similarly, we can show that
∠B = 90°, ∠D = 90°
∴ ∠A = ∠B = ∠C = ∠D = 90°
∴ ABCD is a rectangle.
If the non-parallel sides of a trapezium are equal, prove that it is cyclic. Prove that an isosceles trapezium is cyclic.