Ta có:
\(\widehat{MIB}+\widehat{NIB}=180^o\) (hai góc kề bù)
Mà: \(\widehat{NIB}=\dfrac{1}{5}\widehat{MIB}\) thay vào ta có:
\(\widehat{MIB}+\dfrac{1}{5}\widehat{MIB}=180^o\)
\(\Rightarrow\dfrac{6}{5}\widehat{MIB}=180^o\)
\(\Rightarrow\widehat{MIB}=180^o:\dfrac{6}{5}=150^o\)
\(\Rightarrow\widehat{NIB}=\dfrac{1}{5}\cdot\widehat{MIB}=\dfrac{1}{5}\cdot150^o=30^o\)
Lại có: \(\widehat{NIB}\) đối đỉnh \(\widehat{MIA}\)
\(\Rightarrow\widehat{MIA}=\widehat{NIB}=30^o\)
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