Đáp án:

`a)`

 `{:(S in (SAB)nn(alpha)),(A in (SAB)nn(alpha)):}}`

`=>SA=(SAB)nn(alpha)`

`b)`

`@`Trong `(ABCD):` gọi `O=ACnnBD`

Mà `AC sub (SAC),BD sub (SBD)`

`{:(=>O in (SAC)nn(SBD)),("Mà "S in (SAC)nn(SBD)):}}`

`=>SO=(SAC)nn(SBD)`

`c)`

`@`Trong `(ABCD):` gọi `E=ABnnCD`

Mà `AB sub (SAB),CD sub (SCD)`

`{:(=>E in (SAB)nn(SCD)),("Mà "S in (SAB)nn(SCD)):}}`

`=>SE=(SAB)nn(SCD)`

`d)`

`@`Trong `(ABCD):` gọi `F=ADnnBC`

Mà `AD sub (SAD),BC sub (SBC)`

`{:(=>F in (SAD)nn(SBC)),("Mà "S in (SAD)nn(SBC)):}}`

`=>SF=(SAD)nn(SBC)`