Ta có: `\hat{A_1}+\hat{A_4}=180^o` (`2` góc kề bù)
Vì `3\hat{A_1}=7\hat{A_4}=>\hat{A_1}=7/3\hat{A_4}`
`=>7/3\hat{A_4}+\hat{A_4}=180^o`
`=>10/3\hat{A_4}=180^o`
`=>\hat{A_4}=54^o`
`=>\hat{A_1}=7/3 .54^o =126^o`
`@\hat{A_1}=\hat{A_3}` (`2` góc đối đỉnh) `=>\hat{A_3}=126^o`
`@\hat{A_2}=\hat{A_4}` (`2` góc đối đỉnh) `=>\hat{A_4}=54^o`
A1 = 7/3 A4
A1+A4 = 180°
=> A1 = 7/3 x (180° - A1) = 126°
A1 = A3 = 126°
A2 = A4 = 54°