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`⊕ ∠AOM; ∠BOM` là `2` góc kề bù
`=> ∠BOM + ∠AOM = 180^0`
`=> ∠BOM = 180^0 - ∠AOM = 180^0 - 60^0 = 120^0`
`⊕ OM ⊥ ON => ∠ MON = 90^0`
Tia `ON ∈ ∠BOM => ∠BON + ∠MON = ∠BOM`
`=> ∠BON = ∠BOM - ∠MON = 120^0 - 90^0 = 30^0`
`⊕ 30^0 = {60^0}/2 => ∠BON = 1/2 *∠AOM.`