Vì: `Ax` // `By` `<=>` $\widehat{xAB}$ `+` $\widehat{ABy}$ `= 180^0`
`=>` $\widehat{ABy}$ `= 180^0 -` $\widehat{xAB}$ `= 180^0 - 135^0 = 45^0`
`Cz` // `By` `<=>` $\widehat{yBC}$ `=` $\widehat{zCt}$ `= 45^0` ( đồng vị nhau )
`=>` $\widehat{ABC}$ `=` $\widehat{ABy}$ `+` $\widehat{BCy}$ `= 45^0 + 45^0 = 90^0`
@ Rin