Vì \(\widehat{i}=\widehat{i'}\) \(\Rightarrow\widehat{I_2}=\widehat{I_3}=60^o\)
\(\Rightarrow\widehat{I_3}=\widehat{J_3}=60^o\) (2 góc so le trong)
Vì \(\widehat{i}=\widehat{i'}\) \(\Rightarrow\widehat{J_2}=\widehat{J_3}=60^o\)
Mà \(\widehat{J_3}+\widehat{J_4}=90^o;\widehat{J_3}=60^o\)
\(\Rightarrow\widehat{J_4}=90^o-60^o\)
\(\Rightarrow\widehat{J_4}=30^o\)
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