The joint probability density function $f_{XY}(x, y)$ is defined such that the double integral over the entire plane is equal to 1. That is,
\[
\int_{-\infty}^{\infty}\int_{-\infty}^{\infty}f_{XY}(x, y) \, dx \, dy = 1
\]
However, the function is only non-zero in
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