ABC and ADC are two right triangles with common hypotenuse AC. Prove that ∠ CAD = ∠ CBD.
Required: To find ∠BCD.
Determination: ∠BDC = ∠BAC (= 30°)
| Angles in the same segment
Now, in ∆BCD,
∠BCD + ∠BDC + ∠DBC = 180°
| ∵ The sum of the three angles of a ∆ is 180°
⇒ ∠BCD + 30° + 70° = 180°
⇒ ∠BCD + 100° = 180°
⇒ ∠BCD = 180° - 100° = 80°.