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Draw any shape. From the fact that any magnetic field line that passes the shape must come out, This mean that the number of magnetic field lines going in and out is the same thus the law is proven.

And I have actually seen this proof in a couple of sites, But it doesn't makes sense to me. Magnetic field lines are just the tangent to the magnetic field at any point. So for example draw a shape when a magnetic field cross it for the first time there is a flux with angle theta and there is a B magnetic field, When you cross from the other way, There will be another angle and another value B (If the field changes inside the shape). So clearly for one line the net flux is not always zero.

What I did to fix this is to consider an infinitesimal cube where the magnitude and the direction of the magnetic field is constant then you can find that the net flux is zero and you can build any shape out of these cubes

Is there is something I said wrong? What is with the magnetic field lines proof?