Từ B kẻ \(c\backslash\backslash a\)
\(\Leftrightarrow\widehat{B1}+\widehat{A}=180^0\) (trong cùng phía)
Mà \(\widehat{A}=135^0\Leftrightarrow\widehat{B1}=45^0\)
\(\widehat{B1}+\widehat{B2}=\widehat{ABC}\) (Bc nằm giữa Aa và Cb)
Mà \(\widehat{B1}=45^0;\widehat{ABC}=85^0\)
\(\Leftrightarrow\widehat{B2}=40^0\)
Ta có :
\(\left\{{}\begin{matrix}a\backslash\backslash b\\a\backslash\backslash c\end{matrix}\right.\) \(\Leftrightarrow b\backslash\backslash c\)
\(\Leftrightarrow\widehat{B1}=\widehat{C}\)
Mà \(\widehat{B1}=40^0;\widehat{C}=x\)
\(\Leftrightarrow x=40^0\)
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ABH=
ACH.
