Bài 2.
a1) Ta có: \(\left\{{}\begin{matrix}AD\perp AB\left(gt\right).\\BC\perp AB\left(gt\right).\end{matrix}\right.\) \(\Rightarrow AD//BC\) (Từ vuông góc đến song song).
b1) Vì AD // BC (cmt) \(\Rightarrow\widehat{D}+\widehat{C}=180^o\) (Trong cùng phía).
Mà \(\widehat{C}=60^o\left(gt\right).\)
\(\Rightarrow\) \(\widehat{D}=180^o-60^o=120^o.\)
a2) Ta có: \(\left\{{}\begin{matrix}c\perp a\left(gt\right).\\d\perp a\left(gt\right).\end{matrix}\right.\)\(\Rightarrow c//d\) (Từ vuông góc đến song song).
b2) Vì c // d (cmt) \(\Rightarrow\widehat{P}=\widehat{Q_1}\left(SLT\right).\)Mà \(\widehat{P}=55^o\left(gt\right).\)
\(\Rightarrow\) \(\widehat{Q_1}=55^o.\)
Mà: \(\widehat{Q_1}+\widehat{Q_2}=180^o.\)
\(\Rightarrow\widehat{Q_2}=125^o.\)
