Proving that the shortest distance between two parabolas is along their common normal.

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Can someone prove that the minimum distance between two non-intersecting parabolas is along their common normal (without calculus)? (If I understand how to prove it, I'll prove it myself for other curves too.)

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Assuming the existence of a shortest distance, let the shortest distance join points $A$ and $B$ and construct a circle with diameter $AB$. If the circle cuts either parabola at a point other than $A$ or $B$ there is a line joining the two curves which is shorter than the diameter. Else the circle must be tangent at points $A$ and $B$ and the diameter $AB$ is normal to both curves (because the diameter is perpendicular to the tangent).

There are some implicit assumptions here, but would this do? Note that if $A=B$ then the diameter doesn't have a fixed direction and the situation is different.