Do ABCD là hình thang \(\Rightarrow\widehat{A}+\widehat{D}=180^0\)
\(\Rightarrow3\widehat{D}+\widehat{D}=180^0\)
\(\Rightarrow\widehat{D}=45^0\)
\(\Rightarrow\widehat{A}=3\widehat{D}=135^0\)
Tương tự ta có \(\widehat{B}+\widehat{C}=180^0\)
Mà \(\widehat{B}-\widehat{C}=30^0\Rightarrow\widehat{B}=\widehat{C}+30^0\)
\(\Rightarrow\widehat{C}+30^0+\widehat{C}=180^0\)
\(\Rightarrow2\widehat{C}=150^0\)
\(\Rightarrow\widehat{C}=75^0\)
\(\Rightarrow\widehat{B}=75^0+30^0=105^0\)
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