Hình:
Giải:
Xét tam giác ABC, có:
\(\widehat{A}+\widehat{B}+\widehat{C}=180^0\)(Tổng ba góc tam giác)
\(\Leftrightarrow90^0+\widehat{B}+\widehat{C}=180^0\)
\(\Leftrightarrow\widehat{B}+\widehat{C}=90^0\)
\(\Leftrightarrow\left\{{}\begin{matrix}\widehat{B}< 90^0\\\widehat{C}< 90^0\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}\widehat{B}\ne90^0\\\widehat{C}\ne90^0\end{matrix}\right.\)
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