Ta có: \(\left\{{}\begin{matrix}m\perp CD\\n\perp CD\end{matrix}\right.\\ \Rightarrow m//n\left(dli\right)\)
\(\Rightarrow A+B=180\left(2.goc.kb\right)\\ \Rightarrow B=180-A=180-120=60^o\)
a) Theo hình vẽ ta có:
\(\left\{{}\begin{matrix}m\perp CD\left(gt\right)\\n\perp CD\left(gt\right)\end{matrix}\right.\)
=> \(m\) // \(n\) (từ vuông góc đến song song).
b) Ta có: \(\widehat{CAB}+\widehat{ABD}=180^0\) (vì 2 góc trong cùng phía)
=> \(120^0+\widehat{ABD}=180^0\)
=> \(\widehat{ABD}=180^0-120^0\)
=> \(\widehat{ABD}=60^0\)
Vậy \(\widehat{ABD}=60^0.\)
Chúc bạn học tốt!
