Câu 9.
$\widehat{{{O}_{1}}}=\widehat{{{O}_{3}}}$ ; $\widehat{{{O}_{2}}}=\widehat{{{O}_{4}}}$ (hai góc đối đỉnh)
$\widehat{{{O}_{1}}} + \widehat{{{O}_{2}}}$ = 180o (hai góc kề bù)
$\widehat{{{O}_{2}}} + \widehat{{{O}_{3}}}$ = 180o (hai góc kề bù)
$\widehat{{{O}_{3}}} + \widehat{{{O}_{4}}}$ = 180o (hai góc kề bù)
$\widehat{{{O}_{4}}} + \widehat{{{O}_{1}}}$ = 180o (hai góc kề bù)