Đáp án:

Câu `7.`

`a)`

`@`Trong `(SAB):` gọi `E=IK nn AB`

Mà `{(IK sub (IJK)),(AB sub (ABCD)):}`

`{:(=>E in (IJK) nn (ABCD)),("Mà "J in (IJK) nn (ABCD)):}}`

`=>EJ = (IJK) nn (ABCD)`

`b)`

`@`Trong `(ABCD):` gọi `O=EJ nn BD`

Mà  `{(EJ sub (IJK)),(BD sub (SBD)):}`

`{:(=>O in (IJK) nn (SBD)),("Mà "K in (IJK) nn (SBD)):}}`

`=>OK = (IJK) nn (SBD)` 

`c)`

`@`Trong `(ABCD):` gọi `H=EJ nn BC`

Mà  `{(EJ sub (IJK)),(BC sub (SBC)):}`

`{:(=>H in (IJK) nn (SBC)),("Mà "K in (IJK) nn (SBC)):}}`

`=>HK = (IJK) nn (SBC)` 

Câu `8.`

`a)`

`@`Trong `(ABCD):` gọi `O=AC nn BD`

Mà `{(AC sub (SAC)),(BD sub (SBD)):}`

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

`=>SO = (SAC) nn (SBD)` 

`b)`

`@`Ta có:

`{:(O in (SAC) nn (MBD)),(M in (SAC) nn (MBD)):}}`

`=>OM = (SAC) nn (MBD)`

`c)`

`@`Trong `(ABCD):` gọi `I=BC nn AD`

Mà `{(BC sub (MBC)),(AD sub (SAD)):}`

`{:(=>I in (MBC) nn (SAD)),("Mà "M in (MBC) nn (SAD)):}}`

`=>MI=(MBC) nn (SAD)` 

`d)`

`@`Trong `(ABCD):` gọi `E=AB nn BC`

Mà `{(AB sub (SAB)),(BC sub (SBC)):}`

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

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