Cho

- Hướng dẫn giải

a) $\widehat{mOn}$ =  $\widehat{mAt}$ = 86° => At // On.

b) $\left\{\begin{array}{l}AH\perp On\\ At//On\end{array}\right.=>AH\perp At$

c) $\widehat{OHA}$ = 180° - 90° - 86° = 4°.

d) d $\perp$AH, At $\perp$AH => d // At

=> $\widehat{OBI}=\widehat{OAt}$ ( hai góc đồng vị)