$\int_{\{x:f(x)>k\}}f(x)dx=c$

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How is to find the value of $k$ for the following equation?

$$\int_{\{x:f(x)>k\}}f(x)\,dx=c,$$

where $c$ is a constant. Can anyone please show me a toy example from which I can understand how is to obtain the value of $k$?

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In general, this is a hard problem. Consider the graph below:

enter image description here

You're essentially asking to find the height of the horizontal line that makes the blue area equal to a given amount. Doing so will typically require:

  • Showing that the integral converges;

  • Finding a closed form for the integral;

  • Being able to invert that closed form to solve for $k$.

None of these are guaranteed for a general function. If you at least have convergence then you can use numeric methods to find an approximation, but even then it's likely to be difficult.