Vì `a` // `b`
`=>` $\widehat{B2}$ = $\widehat{B1}$ `( 2` góc đối đỉnh `)`
Thay $\widehat{B1}$ = $75^\circ$
`=>` $\widehat{B2}$ = $75^\circ$
Vì `a` // `b`
`=>` $\widehat{B1}$ = $\widehat{A2}$ `( 2` góc đồng vị `)`
Thay $\widehat{B1}$ = $75^\circ$
`=>` $\widehat{A2}$ = $75^\circ$
Vì `a` // `b`
`=>` $\widehat{A1}$ + $\widehat{B1}$ = $180^\circ$ `( 2` góc trong cùng phía `)`
Thay $\widehat{B1}$ = $75^\circ$
`=>` $\widehat{A1}$ + $75^\circ$ = $180^\circ$
`=>` $\widehat{A1}$ = $180^\circ$ - $75^\circ$
`=>` $\widehat{A1}$ = $105^\circ$
Vì `a` // `b`
`=>` $\widehat{A1}$ = $\widehat{A3}$ `( 2` góc đối đỉnh `)`
Thay $\widehat{A1}$ = $105^\circ$
`=>` $\widehat{A3}$ = $105^\circ$
