`a)` $\fbox{Đúng}$

`b)`

`MN:{("đi qua"\ M(1; -1;0)),(vec(MN)=(-3; 1; 4)=>vec(u_1)=(3; -1; -4)):}`

`=>` `MN:(x-1)/3=(y+1)/(-1)=z/(-4)`

`=>` $\fbox{Đúng}$

`c)`

Ta có: `{:(vec(u_(MN))=(-3;1;4); vec(M_1)=(1; -1; 0)),(vec(u_(d))=(3;-1;-4); vec(M_1)=(-5; 1; 8)):}}`

Xét: `{( [vec(u_(MN)), vec(u_d)]=0),([vec(u_(MN))",vec(M_1M_2)]=0):}`

`=>` `MN equiv d`

`=>` $\fbox{Sai}$

`d)`

Đặt `alpha=(MN, Ox)`

Ta có: `vec(u_(Ox))=(1;0;0)`

`=>` `cos alpha=(|(-3)*1+1*0+4*0|)/(sqrt((-3)^2+1^2+4^2)*sqrt(1^2+0^2+0^2))=(3sqrt(26))/26`

`=>` $\fbox{Sai}$