The marginal pdf of $X_{1}$, $f_{X_{1}}(x_{1})$, is obtained by integrating the joint pdf over all possible values of $x_{2}$:
$$
f_{X_{1}}(x_{1}) = \int_{0}^{1} f(x_{1}, x_{2}) dx_{2} = \int_{0}^{1} 12x_{1}x_{2}(1-x_{2}) dx_{2}
$$
Similarly, the marginal pdf
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