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arbri

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## Homework Statement

In "Methods of theoretical physics" from the authors Morse and Feshbach is writen:

" The displacement (dx,dy,dz) is perpendicular to the surface if the component displacemetn satisfy the equation:

[Broken]

How to prove this relation?

## Homework Equations

the relevant equation is the equation of total change

## The Attempt at a Solution

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