# Normal lines

arbri

## 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:

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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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$$\frac{dx}{\partial \psi /\partial x} = \frac{dy}{\partial \psi /\partial y} = \frac{dz}{\partial \psi /\partial z}$$