`#Mon`
`\text{ Có CM là phân giác}` `\hat{BCD}\text{(gt)}`
`=>\hat{C_{1}}=\frac{1}{2}\hat{BCD}(\text{đ/l})`
`\text{ Có DM là phân giác}` `\hat{ADC}\text{(gt)}`
`=>\hat{D_{1}}=\frac{1}{2}\hat{ADC}(\text{đ/l})`
`\text{ Lại có:}` `\hat{C_{1}}+\hat{D_{1}}+\hat{CMD}=180^o(\text{đ/l})`
`=>\frac{1}{2}\hat{BCD}+\frac{1}{2}\hat{ADC}+105^o=180^o`
`=>\frac{1}{2}(\hat{BCD}+\hat{ADC})=75^o`
`=>\hat{BCD}+\hat{ADC}=150^o`
`\text{ Xét tứ giác ABCD, có:}`
`\hat{A}+\hat{B}+\hat{BCD}+\hat{ADC}=360^o`
`=>\hat{A}+\hat{B}+150^o=360^o`
`=>\hat{A}+\hat{B}=210^o`
`Mà` `\hat{A}-\hat{B}=60^o`
`\text{ Ta cộng vế:}`
`2\hat{A}=270^o`
`=>\hat{A}=135^o`
`=>135^o +\hat{B}=210^o`
`=>\hat{B}=75^o`
