Hôm qua mới chỉ xong :vv
Xét ΔABC có:
\(\widehat{A}+\widehat{B}=120^o\Rightarrow\widehat{A}=120^o-\widehat{B}\left(1\right)\)
\(\widehat{A}-\widehat{B}=30^o\Rightarrow\widehat{A}=30^o+\widehat{B}\left(2\right)\)
Từ \(\left(1\right);\left(2\right)\Rightarrow120^o-\widehat{B}=30^o+\widehat{B}\Leftrightarrow2\widehat{B}=120^o-30^o\Leftrightarrow\widehat{B}=\frac{90^o}{2}=45^o\)
Theo (1) ta có: \(\widehat{A}=120^o-\widehat{B}=120^o-45^o=75^o\)
Ta có: \(\widehat{A}+\widehat{B}+\widehat{C}=180^o\left(d/li\right)\Rightarrow\widehat{C}=180^o-\widehat{A}-\widehat{B}=180^o-45^o-75^o=60^o\)
Trong ΔABC có: \(\widehat{B}< \widehat{C}< \widehat{A}\left(45^o< 60^o< 75^o\right)\)
\(\Rightarrow AC< AB< BC\left(d/li\right)\)
