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Có:
`+,\hat{xOy}+\hat{x'Oy}=180°`
`=>120°+\hat{x'Oy}=180°` 
`<=>\hat{x'Oy}=180°-120°` 
`<=>\hat{x'Oy}=60°` (Kề bù)
`+,Ot` là phân giác `\hat{xOy}`
`=>\hat{xOt}=\hat{tOy}=\hat{xOy}/2=120°/2=60°`
`+,Ot'` là phân giác `\hat{x'Oy}`
`=>\hat{x'Ot'}=\hat{t'Oy}=\hat{xOy}/2=60°/2=30°`
`+,\hat{tOt'}=\hat{t'Oy}+\hat{yOt}`

`=>\hat{tOt'}=30°+60°`

`<=>\hat{tOt'}=90°`

Vậy `\hat{x'Ot}=30°,\hat{tOt'}=90°`