`3)`
`a)\hat{MNE}=\hat{MEF}(=30^o )`
`=>MN //// EF`
~~~~~~~~~~~
`b)\hat{DKH}=\hat{DFE} (=60^o )`
`=>HK //// EF`
~~~~~~~~~~
`c){:(MN //// EF),(HK //// EF):}}=>MN //// HK`
________________________________
`4)`
`Ax //// Dy=>{(\hat{BAD}+\hat{ADC}=180^o ),(\hat{ABC}=\hat{BCy}):}`
`=>{(\hat{ADC}=180^o -90^o =90^o ),(\hat{ABC}=50^o ):}`
Ta có: `\hat{ABC}+\hat{CBx}=180^o`
`=>\hat{CBx}=180^o -50^o =130^o `
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