X and Y are independent if and only if their joint pdf can be written as the product of their marginal pdfs, i.e.,
$$f(x, y) = f_X(x) f_Y(y)$$
Now, let's consider the conditional pdf $f_{X|Y}(x|y)$. By definition, we have:
$$f_{X|Y}(x|y) = \frac{f(x,
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