- AD tangent to circle with center O
- C on this circle
- => ∠BAD= ∠BCA
- Demonstration
- Define E in the circle such as E, O and A are colinear
- ∠0AD=90° and ∠ECA= 90°
- ∠BAE intercepts arc BE and ∠BCE intercepts arc BE => ∠BAE= ∠BCE
- ∠0AD= ∠0AB+∠BAD=90°
- ∠ECA= ∠ECB+∠BCA=90°
- And ∠BAE= ∠BCE
- Therefore ∠BAD= ∠BCA