The marginal density of \( X \), \( f_X(x) \), is found by integrating the joint pdf over all values of \( y \):
\[
f_X(x) = \int_0^1 (x + y) \, dy
\]
\[
= \int_0^1 x \, dy + \int_0^1 y \, dy
\]
\[
= x \cdot 1 + \left[\frac{y^2}{2}\right]_0^1
\]
\[
= x +
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