\begin{array}{c}\color{#003366}{-Gw}\color{#005A9C}{en}\color{#007ACC}{St}\color{#00AEEF}{ac}\color{#00CFFF}{y}\color{#A4D3EE}{-}\color{#D3D3D3}{}\color{#E0E0E0}{}\end{array} 

Bài `7`:

Xét tứ giác `ABCD` có:

`hat{C} + hat{D} + hat{A} + hat{DBA} = 360^o`

`=> hat{DBA} = 360^o - 70^o - 90^o xx 2 = 110^o`

Lại có:

`hat{MBE} + hat{DBE} + hat{DBA} = 180^o`

`=> hat{MBE} = 180^o - 110^o - 40^o = 30^o`

Kẻ `MK` // `EB`

`=> hat{EMB} = hat{MBK}` (`2` góc so le trong)

`=> hat{MBK} = 30^o`

`=> hat{KBN} = 50^o - 30^o = 20^o`

Có `MK` // `SN`

`=> hat{KBN} = hat{MNS} (2` góc so le trong)

`=> hat{MNS} = 20^o`