Ta có: `\hat{A_1}+\hat{A_2}=180^o` (`2` góc kề bù)
Mà `\hat{A_2}-\hat{A_1}=30^o =>\hat{A_2}=30^o +\hat{A_1}`
`=>\hat{A_1}+30^o +\hat{A_1}=180^o`
`=>2\hat{A_1}=150^o`
`=>\hat{A_1}=75^o`
`=>\hat{A_2}=30^o +75^o =105^o`
`@\hat{A_1}=\hat{A_3}` (`2` góc đối đỉnh) `=>\hat{A_3}=75^o`
`@\hat{A_2}=\hat{A_4}` (`2` góc đối dỉnh) `=>\hat{A_4}=105^o`