`@Mon`
`\triangleAOC` `có:`
`OA=OC=RR; AC=RR\sqrt{2}`
`=>\triangleAOC` `\text{ vuông cân tại O(định lý Py-ta-go đảo)}`
`=>\hat{OAC}=\hat{OCA}=45^o`
`Kẻ` `OI \bot AB` `\text{ tại I thì}` `AI=\frac{1}{2}AB=\frac{R\sqrt{3}}{2}`
`=>OI=\sqrt{OA^2-AI^2}=\sqrt{RR^2-(\frac{RR\sqrt{3}}{2})^2}=\frac{RR}{2}`
`=>\hat{OAI}=30^o`
`=>\hat{BAC}=45^o +30^o=75^o`
`\text{ Ta có:}`
`2\hat{OBC}=\hat{OBC}+\hat{OCB}`
`=\hat{ABC}-\hat{B_1}+\hat{ACB}-\hat{C_2}`
`=\hat{ABC}+\hat{ACB}-(\hat{B_1}+\hat{C_2})`
`=(180^o -\hat{BAC})-(\hat{A_1}+\hat{A_2})`
`=180^o -75^o -75^o`
`=30^o`
`=>\hat{OBC}=\hat{OCB}=30^o:2=15^o`
`=>\hat{ABC}=30^o +15^o=45^o`
`Và` `\hat{ACB}=45^o +15^o=60^o`
