Marginal Pdfs 8A73C6
1. **Problem statement:** Given the joint probability density function (pdf) $$f(x,y) = \begin{cases} \frac{3}{2} y^2 & 0 < x < 2, 0 < y < 1 \\ 0 & \text{otherwise} \end{cases}$$
Find:
(a) The marginal pdf of $X$.
(b) The marginal pdf of $Y$.
2. **Recall the formulas:**
- The marginal pdf of $X$ is found by integrating the joint pdf over all values of $y$:
$$f_X(x) = \int_{-\infty}^{\infty} f(x,y) \, dy$$
- The marginal pdf of $Y$ is found by integrating the joint pdf over all values of $x$:
$$f_Y(y) = \int_{-\infty}^{\infty} f(x,y) \, dx$$
3. **Find $f_X(x)$:**
Since $f(x,y) = \frac{3}{2} y^2$ for $0 < x < 2$ and $0 < y < 1$, and zero otherwise,
$$f_X(x) = \int_0^1 \frac{3}{2} y^2 \, dy = \frac{3}{2} \int_0^1 y^2 \, dy$$
Calculate the integral:
$$\int_0^1 y^2 \, dy = \left[ \frac{y^3}{3} \right]_0^1 = \frac{1}{3}$$
So,
$$f_X(x) = \frac{3}{2} \times \frac{1}{3} = \frac{1}{2}$$
This holds for $0 < x < 2$, and $f_X(x) = 0$ otherwise.
4. **Find $f_Y(y)$:**
Similarly,
$$f_Y(y) = \int_0^2 \frac{3}{2} y^2 \, dx = \frac{3}{2} y^2 \int_0^2 dx = \frac{3}{2} y^2 \times 2 = 3 y^2$$
This holds for $0 < y < 1$, and $f_Y(y) = 0$ otherwise.
5. **Summary:**
- Marginal pdf of $X$:
$$f_X(x) = \begin{cases} \frac{1}{2} & 0 < x < 2 \\ 0 & \text{otherwise} \end{cases}$$
- Marginal pdf of $Y$:
$$f_Y(y) = \begin{cases} 3 y^2 & 0 < y < 1 \\ 0 & \text{otherwise} \end{cases}$$