a) Ta có:
\(\left\{{}\begin{matrix}a\perp c\\b\perp c\end{matrix}\right.\)
\(\Rightarrow a//b\)
b) Ta có:
\(\widehat{C_1}=\widehat{B_1}\)
Hai góc đồng vị
\(\Rightarrow\widehat{B_1}=75^o\)
Mà: \(\widehat{C_1}=\widehat{B_3}\)
Hai góc so le trong
\(\Rightarrow\widehat{B_3}=75^o\)
Mà: \(\widehat{B_3}+\widehat{B_4}=180^o\)
Hai góc kề bù
\(\Rightarrow\widehat{B_4}=180^o-\widehat{B_3}=180^o-75^o=105^o\)
Mà: \(\widehat{B_4}=\widehat{B_2}\)
Hai góc đối đỉnh
\(\Rightarrow\widehat{B_2}=105^o\)
a: a vuông góc AD
b vuông góc AD
=>a//b
b: a//b
=>góc C1=góc B3(hai góc so le trong)
=>góc B3=75 độ
góc B1=góc B3=75 độ
góc B2=góc B4=180-75=105 độ
