ABC and ADC are two right triangles with common hypotenuse AC. Prove that ∠ CAD = ∠ CBD.
Given: ABCD is a cyclic trapezium with AD || BC. ∠B = 70°.
To determine: Other three angles of the trapezium.
Determination:
∠B + ∠D = 180°
| ∵ Opposite angles of a cyclic quadrilateral are supplementary
⇒ 70° + ∠D =180°
⇒ ∠D = 180° - 70°
⇒ ∠D = 110°
Again, ∵ AD || BC and transversal AB intersects them
∵ ∠A + ∠B = 180°
| ∵ The sum of the consecutive interior angles on the same side of a transversal is 180°
⇒ ∠A + 70° = 180°
⇒ ∠A = 180° - 70°
⇒ ∠A = 110°
Also, ∠A + ∠C = 180°
| ∵ Opposite angles of a cyclic quadrilateral are supplementary
⇒ 110° + ∠C = 180°
⇒ ∠C = 180° - 110°
⇒ ∠C = 70°.