Partial Differential Equations : Linear & Quasi-Linear PDEs

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Example Questions

Example Question #3 : Partial Differential Equations

Determine if the statement is true or false:

If the \(\displaystyle z\)-axis is the axis of symmetry and a surface is revolving around it and \(\displaystyle f\) is an arbitrary function, then the partial differential equation associated with that said surface, satisfies the equation:

\(\displaystyle z=f(x^2+y^2)\)

Possible Answers:

False

True

Correct answer:

True

Explanation:

To determine the truth of this statement, assume the following.

\(\displaystyle f\) is some function 

\(\displaystyle u=x^2+y^2\)

From here, differential \(\displaystyle u\) with respect to \(\displaystyle x\) and \(\displaystyle y\).

\(\displaystyle z=f(x^2+y^2)\)

\(\displaystyle \\\frac{dz}{dx}=2x\cdot f'(u) \\\\\frac{dz}{dy}=2y\cdot f'(u)\)

Next eliminate \(\displaystyle f'(u)\) as it is an arbitrary function.

This leads to the result,

\(\displaystyle y\cdot \frac{dz}{dx}-x\cdot \frac{dy}{dz}=0\)

Therefore, the statement,

If the \(\displaystyle z\)-axis is the axis of symmetry and a surface is revolving around it and \(\displaystyle f\) is an arbitrary function, then the partial differential equation associated with that said surface, satisfies the equation:

\(\displaystyle z=f(x^2+y^2)\)

is true.

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