Finding a probability density function of a random variable given a relation between another random variable

probability distributionsstatistics

I am trying to find a pdf for a random variable $X$ where $X=-2Y+1$ and $Y$ is given by $N(4,9)$

Here is my attempt:

we know $\mu=4$ and $\sigma=3$. so that the normal distribution of $Y$ is given by $\frac{1}{3\sqrt{2\pi}}e^\frac{-(y-4)^2}{18}$
We can differentiate the cumulative function of $X$ to get the pdf for $X$.
cdf of $X = P(X<x)$ = $P(-2Y+1<x)=P(Y<\frac{-(x-1)}{2})=\int_{-\infty}^{\frac{-(x-1)}{2}}\frac{1}{3\sqrt{2\pi}}e^\frac{-(y-4)^2}{18}dy$
so $\frac{d}{dx}(\int_{-\infty}^{\frac{-(x-1)}{2}}\frac{1}{3\sqrt{2\pi}}e^\frac{-(y-4)^2}{18}dy)=f(x)$, which is the density function for $X$
$f(x)=-\frac{1}{6\sqrt{2\pi}}e^\frac{-(\frac{-(x-1)}{2}-4)^2}{18}$

Is this a correct way to approach the problem? I feel like my answer is very funky.

Best Answer

Let us denote distribution functions by $F$, and density functions by $f$. Then, \begin{align*} F_X(x) &= P(X \leqslant x)\\ &= P\left(Y \geqslant \frac{1-x}{2}\right)\\ &= 1 - F_Y\left(\frac{1-x}{2}\right)\\ &= 1 - \int_{-\infty}^{\frac{1-x}{2}} f_Y(t) dt\\ &= 1 - \int_{0}^{\frac{1-x}{2}} f_Y(t) dt - \int_{-\infty}^{0} f_Y(t) dt\\ &= \frac{1}{2} - \int_{0}^{\frac{1-x}{2}} f_Y(t) dt \end{align*} Therefore, by Fundamental Theorem of Calculus, we have : $$ f_X(x) = \frac{1}{2} \cdot f_Y\left(\frac{1-x}{2}\right) = \frac{1}{6\sqrt{2\pi}} \exp\left[-\frac{(x+7)^2}{72}\right]$$ So, $~X \sim N(-7,36)$ . Hope it helps.

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