The angle t between two lines is the angle between two direction vectors of the lines.
Take line a with direction vector u and line b with direction vector v.

That is:
u.v= ||u||.||v||.cos(t)
and then:

The angle between two planes is given by the angle between the normal vectors,
n₁ and n₂, that is:

Let's name t to the angle which line makes with plane, v the direction vector of line and n the normal vector of the plane.

Then: