Proof that if $Y=2X+3$ then $f_Y(y)=frac12f_X(frac{y-3}2)$
$begingroup$
I want to prove that if probability density function (PDF) of a random variable $X$ is $f_X(x)$ and another random variable $Y$ is defined as $Y=2X+3$, then the PDF of $Y$ is $f_Y(y)=frac{1}{2} f_X(frac{y-3}{2})$.
My Approach:
We Know: $P(X leq x)= int_{-infty}^x f_X(x) , dx$
similarly,$$P(Y leq y)= int_{-infty}^y f_Y(y) , dy$$
Now,$, Y=2X+3$
$$implies P(Y leq y)= P((2X+3) leq y)$$
$$=P(X leq (frac{y-3}{2}))$$
$$implies P(Y leq y)=int_{-infty}^{frac{y-3}{2}} f_X(y) , dy$$
$$implies int_{-infty}^y f_Y(y) , dy=int_{-infty}^{frac{y-3}{2}} f_X(y) , dy $$
Now how can I proceed further to find a relationship between $f_Y$ and $f_X $?
probability-theory probability-distributions
$endgroup$
|
show 2 more comments
$begingroup$
I want to prove that if probability density function (PDF) of a random variable $X$ is $f_X(x)$ and another random variable $Y$ is defined as $Y=2X+3$, then the PDF of $Y$ is $f_Y(y)=frac{1}{2} f_X(frac{y-3}{2})$.
My Approach:
We Know: $P(X leq x)= int_{-infty}^x f_X(x) , dx$
similarly,$$P(Y leq y)= int_{-infty}^y f_Y(y) , dy$$
Now,$, Y=2X+3$
$$implies P(Y leq y)= P((2X+3) leq y)$$
$$=P(X leq (frac{y-3}{2}))$$
$$implies P(Y leq y)=int_{-infty}^{frac{y-3}{2}} f_X(y) , dy$$
$$implies int_{-infty}^y f_Y(y) , dy=int_{-infty}^{frac{y-3}{2}} f_X(y) , dy $$
Now how can I proceed further to find a relationship between $f_Y$ and $f_X $?
probability-theory probability-distributions
$endgroup$
1
$begingroup$
Use the change of variable $yto2y+3$ in the very last integral.
$endgroup$
– Did
Jan 8 at 9:56
$begingroup$
if we put $y to 2y +3 $ , then we will get $f_Y(y)=2 f_X(2y +3)$,now how to proceed?
$endgroup$
– Suresh
Jan 8 at 10:12
$begingroup$
No we do not get that.
$endgroup$
– Did
Jan 8 at 14:30
1
$begingroup$
Wrong change of variable, possibly at least partly due to the horrendous practice of using the same symbol for the bound of the integral and for the variable of integration. Somehow, you are trying to get mistakes...
$endgroup$
– Did
Jan 9 at 6:51
1
$begingroup$
You are doing the change of variable, backwards.
$endgroup$
– Did
Jan 10 at 10:38
|
show 2 more comments
$begingroup$
I want to prove that if probability density function (PDF) of a random variable $X$ is $f_X(x)$ and another random variable $Y$ is defined as $Y=2X+3$, then the PDF of $Y$ is $f_Y(y)=frac{1}{2} f_X(frac{y-3}{2})$.
My Approach:
We Know: $P(X leq x)= int_{-infty}^x f_X(x) , dx$
similarly,$$P(Y leq y)= int_{-infty}^y f_Y(y) , dy$$
Now,$, Y=2X+3$
$$implies P(Y leq y)= P((2X+3) leq y)$$
$$=P(X leq (frac{y-3}{2}))$$
$$implies P(Y leq y)=int_{-infty}^{frac{y-3}{2}} f_X(y) , dy$$
$$implies int_{-infty}^y f_Y(y) , dy=int_{-infty}^{frac{y-3}{2}} f_X(y) , dy $$
Now how can I proceed further to find a relationship between $f_Y$ and $f_X $?
probability-theory probability-distributions
$endgroup$
I want to prove that if probability density function (PDF) of a random variable $X$ is $f_X(x)$ and another random variable $Y$ is defined as $Y=2X+3$, then the PDF of $Y$ is $f_Y(y)=frac{1}{2} f_X(frac{y-3}{2})$.
My Approach:
We Know: $P(X leq x)= int_{-infty}^x f_X(x) , dx$
similarly,$$P(Y leq y)= int_{-infty}^y f_Y(y) , dy$$
Now,$, Y=2X+3$
$$implies P(Y leq y)= P((2X+3) leq y)$$
$$=P(X leq (frac{y-3}{2}))$$
$$implies P(Y leq y)=int_{-infty}^{frac{y-3}{2}} f_X(y) , dy$$
$$implies int_{-infty}^y f_Y(y) , dy=int_{-infty}^{frac{y-3}{2}} f_X(y) , dy $$
Now how can I proceed further to find a relationship between $f_Y$ and $f_X $?
probability-theory probability-distributions
probability-theory probability-distributions
edited Jan 8 at 9:58
Did
249k23228467
249k23228467
asked Jan 8 at 9:49
SureshSuresh
371110
371110
1
$begingroup$
Use the change of variable $yto2y+3$ in the very last integral.
$endgroup$
– Did
Jan 8 at 9:56
$begingroup$
if we put $y to 2y +3 $ , then we will get $f_Y(y)=2 f_X(2y +3)$,now how to proceed?
$endgroup$
– Suresh
Jan 8 at 10:12
$begingroup$
No we do not get that.
$endgroup$
– Did
Jan 8 at 14:30
1
$begingroup$
Wrong change of variable, possibly at least partly due to the horrendous practice of using the same symbol for the bound of the integral and for the variable of integration. Somehow, you are trying to get mistakes...
$endgroup$
– Did
Jan 9 at 6:51
1
$begingroup$
You are doing the change of variable, backwards.
$endgroup$
– Did
Jan 10 at 10:38
|
show 2 more comments
1
$begingroup$
Use the change of variable $yto2y+3$ in the very last integral.
$endgroup$
– Did
Jan 8 at 9:56
$begingroup$
if we put $y to 2y +3 $ , then we will get $f_Y(y)=2 f_X(2y +3)$,now how to proceed?
$endgroup$
– Suresh
Jan 8 at 10:12
$begingroup$
No we do not get that.
$endgroup$
– Did
Jan 8 at 14:30
1
$begingroup$
Wrong change of variable, possibly at least partly due to the horrendous practice of using the same symbol for the bound of the integral and for the variable of integration. Somehow, you are trying to get mistakes...
$endgroup$
– Did
Jan 9 at 6:51
1
$begingroup$
You are doing the change of variable, backwards.
$endgroup$
– Did
Jan 10 at 10:38
1
1
$begingroup$
Use the change of variable $yto2y+3$ in the very last integral.
$endgroup$
– Did
Jan 8 at 9:56
$begingroup$
Use the change of variable $yto2y+3$ in the very last integral.
$endgroup$
– Did
Jan 8 at 9:56
$begingroup$
if we put $y to 2y +3 $ , then we will get $f_Y(y)=2 f_X(2y +3)$,now how to proceed?
$endgroup$
– Suresh
Jan 8 at 10:12
$begingroup$
if we put $y to 2y +3 $ , then we will get $f_Y(y)=2 f_X(2y +3)$,now how to proceed?
$endgroup$
– Suresh
Jan 8 at 10:12
$begingroup$
No we do not get that.
$endgroup$
– Did
Jan 8 at 14:30
$begingroup$
No we do not get that.
$endgroup$
– Did
Jan 8 at 14:30
1
1
$begingroup$
Wrong change of variable, possibly at least partly due to the horrendous practice of using the same symbol for the bound of the integral and for the variable of integration. Somehow, you are trying to get mistakes...
$endgroup$
– Did
Jan 9 at 6:51
$begingroup$
Wrong change of variable, possibly at least partly due to the horrendous practice of using the same symbol for the bound of the integral and for the variable of integration. Somehow, you are trying to get mistakes...
$endgroup$
– Did
Jan 9 at 6:51
1
1
$begingroup$
You are doing the change of variable, backwards.
$endgroup$
– Did
Jan 10 at 10:38
$begingroup$
You are doing the change of variable, backwards.
$endgroup$
– Did
Jan 10 at 10:38
|
show 2 more comments
2 Answers
2
active
oldest
votes
$begingroup$
"We know: $P(X leq x)= int_{-infty}^x f_X(x) , dx$... "
Observe that on RHS the $x$ is fixed and variable at the same time. That is confusing notation. You should write something like:$$P(X leq x)= int_{-infty}^x f_X(u) , du$$
You found: $$P(Yleq y)=int_{-infty}^{frac{y-3}2}f(u)du=int_{-infty}^{+infty}mathbf1_{left(-infty,frac{y-3}2right]}(u)f(u)du$$
(Notice that I introduce $u$ here in order to avoid the confusing notation).
Now substitute $u=frac{v-3}2$ to get $$cdots=int_{-infty}^{infty}mathbf1_{left(-infty,frac{y-3}2right]}left(frac{v-3}2right)fleft(frac{v-3}2right)dleft(frac{v-3}2right)=int^y_{-infty}frac12 fleft(frac{v-3}2right)dv$$
$endgroup$
$begingroup$
Sorry , i am not able to understand how you are getting "$int_{-infty}^{+infty}mathbf1_{left(-infty,frac{y-3}2right]}(u)f(u)du$ " from "$int_{-infty}^{frac{y-3}2}f(u)du=$";could you please explain it...
$endgroup$
– Suresh
Jan 8 at 10:22
1
$begingroup$
If $Asubseteqmathbb R$ is measurable then $int_Af(u);du$ is nothing else but a notation for $int_{-infty}^{infty}mathbf1_A(u)f(u)du$. If $A=(a,b]$ then $int_a^bf(u)du$ is a notation for $int_{(a,b]}f(u)du=int_{-infty}^{infty}mathbf1_{(a,b]}(u)f(u)du$. You can take $a=-infty$ here.
$endgroup$
– drhab
Jan 8 at 10:25
add a comment |
$begingroup$
$$F_Y(y) = F_Xleft(frac{y-3}{2} right)$$
Now, let's differentiate with respect to $y$ by chain rule
$$frac{d}{dy}F_Y(y) = frac{d}{dy}F_Xleft(frac{y-3}{2} right)=f_Xleft(frac{y-3}{2} right)frac{d}{dy}left(frac{y-3}{2} right)$$
that is $$f_Y(y)=frac12f_Xleft(frac{y-3}{2} right)$$
$endgroup$
add a comment |
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2 Answers
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2 Answers
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votes
$begingroup$
"We know: $P(X leq x)= int_{-infty}^x f_X(x) , dx$... "
Observe that on RHS the $x$ is fixed and variable at the same time. That is confusing notation. You should write something like:$$P(X leq x)= int_{-infty}^x f_X(u) , du$$
You found: $$P(Yleq y)=int_{-infty}^{frac{y-3}2}f(u)du=int_{-infty}^{+infty}mathbf1_{left(-infty,frac{y-3}2right]}(u)f(u)du$$
(Notice that I introduce $u$ here in order to avoid the confusing notation).
Now substitute $u=frac{v-3}2$ to get $$cdots=int_{-infty}^{infty}mathbf1_{left(-infty,frac{y-3}2right]}left(frac{v-3}2right)fleft(frac{v-3}2right)dleft(frac{v-3}2right)=int^y_{-infty}frac12 fleft(frac{v-3}2right)dv$$
$endgroup$
$begingroup$
Sorry , i am not able to understand how you are getting "$int_{-infty}^{+infty}mathbf1_{left(-infty,frac{y-3}2right]}(u)f(u)du$ " from "$int_{-infty}^{frac{y-3}2}f(u)du=$";could you please explain it...
$endgroup$
– Suresh
Jan 8 at 10:22
1
$begingroup$
If $Asubseteqmathbb R$ is measurable then $int_Af(u);du$ is nothing else but a notation for $int_{-infty}^{infty}mathbf1_A(u)f(u)du$. If $A=(a,b]$ then $int_a^bf(u)du$ is a notation for $int_{(a,b]}f(u)du=int_{-infty}^{infty}mathbf1_{(a,b]}(u)f(u)du$. You can take $a=-infty$ here.
$endgroup$
– drhab
Jan 8 at 10:25
add a comment |
$begingroup$
"We know: $P(X leq x)= int_{-infty}^x f_X(x) , dx$... "
Observe that on RHS the $x$ is fixed and variable at the same time. That is confusing notation. You should write something like:$$P(X leq x)= int_{-infty}^x f_X(u) , du$$
You found: $$P(Yleq y)=int_{-infty}^{frac{y-3}2}f(u)du=int_{-infty}^{+infty}mathbf1_{left(-infty,frac{y-3}2right]}(u)f(u)du$$
(Notice that I introduce $u$ here in order to avoid the confusing notation).
Now substitute $u=frac{v-3}2$ to get $$cdots=int_{-infty}^{infty}mathbf1_{left(-infty,frac{y-3}2right]}left(frac{v-3}2right)fleft(frac{v-3}2right)dleft(frac{v-3}2right)=int^y_{-infty}frac12 fleft(frac{v-3}2right)dv$$
$endgroup$
$begingroup$
Sorry , i am not able to understand how you are getting "$int_{-infty}^{+infty}mathbf1_{left(-infty,frac{y-3}2right]}(u)f(u)du$ " from "$int_{-infty}^{frac{y-3}2}f(u)du=$";could you please explain it...
$endgroup$
– Suresh
Jan 8 at 10:22
1
$begingroup$
If $Asubseteqmathbb R$ is measurable then $int_Af(u);du$ is nothing else but a notation for $int_{-infty}^{infty}mathbf1_A(u)f(u)du$. If $A=(a,b]$ then $int_a^bf(u)du$ is a notation for $int_{(a,b]}f(u)du=int_{-infty}^{infty}mathbf1_{(a,b]}(u)f(u)du$. You can take $a=-infty$ here.
$endgroup$
– drhab
Jan 8 at 10:25
add a comment |
$begingroup$
"We know: $P(X leq x)= int_{-infty}^x f_X(x) , dx$... "
Observe that on RHS the $x$ is fixed and variable at the same time. That is confusing notation. You should write something like:$$P(X leq x)= int_{-infty}^x f_X(u) , du$$
You found: $$P(Yleq y)=int_{-infty}^{frac{y-3}2}f(u)du=int_{-infty}^{+infty}mathbf1_{left(-infty,frac{y-3}2right]}(u)f(u)du$$
(Notice that I introduce $u$ here in order to avoid the confusing notation).
Now substitute $u=frac{v-3}2$ to get $$cdots=int_{-infty}^{infty}mathbf1_{left(-infty,frac{y-3}2right]}left(frac{v-3}2right)fleft(frac{v-3}2right)dleft(frac{v-3}2right)=int^y_{-infty}frac12 fleft(frac{v-3}2right)dv$$
$endgroup$
"We know: $P(X leq x)= int_{-infty}^x f_X(x) , dx$... "
Observe that on RHS the $x$ is fixed and variable at the same time. That is confusing notation. You should write something like:$$P(X leq x)= int_{-infty}^x f_X(u) , du$$
You found: $$P(Yleq y)=int_{-infty}^{frac{y-3}2}f(u)du=int_{-infty}^{+infty}mathbf1_{left(-infty,frac{y-3}2right]}(u)f(u)du$$
(Notice that I introduce $u$ here in order to avoid the confusing notation).
Now substitute $u=frac{v-3}2$ to get $$cdots=int_{-infty}^{infty}mathbf1_{left(-infty,frac{y-3}2right]}left(frac{v-3}2right)fleft(frac{v-3}2right)dleft(frac{v-3}2right)=int^y_{-infty}frac12 fleft(frac{v-3}2right)dv$$
answered Jan 8 at 10:14
drhabdrhab
104k545136
104k545136
$begingroup$
Sorry , i am not able to understand how you are getting "$int_{-infty}^{+infty}mathbf1_{left(-infty,frac{y-3}2right]}(u)f(u)du$ " from "$int_{-infty}^{frac{y-3}2}f(u)du=$";could you please explain it...
$endgroup$
– Suresh
Jan 8 at 10:22
1
$begingroup$
If $Asubseteqmathbb R$ is measurable then $int_Af(u);du$ is nothing else but a notation for $int_{-infty}^{infty}mathbf1_A(u)f(u)du$. If $A=(a,b]$ then $int_a^bf(u)du$ is a notation for $int_{(a,b]}f(u)du=int_{-infty}^{infty}mathbf1_{(a,b]}(u)f(u)du$. You can take $a=-infty$ here.
$endgroup$
– drhab
Jan 8 at 10:25
add a comment |
$begingroup$
Sorry , i am not able to understand how you are getting "$int_{-infty}^{+infty}mathbf1_{left(-infty,frac{y-3}2right]}(u)f(u)du$ " from "$int_{-infty}^{frac{y-3}2}f(u)du=$";could you please explain it...
$endgroup$
– Suresh
Jan 8 at 10:22
1
$begingroup$
If $Asubseteqmathbb R$ is measurable then $int_Af(u);du$ is nothing else but a notation for $int_{-infty}^{infty}mathbf1_A(u)f(u)du$. If $A=(a,b]$ then $int_a^bf(u)du$ is a notation for $int_{(a,b]}f(u)du=int_{-infty}^{infty}mathbf1_{(a,b]}(u)f(u)du$. You can take $a=-infty$ here.
$endgroup$
– drhab
Jan 8 at 10:25
$begingroup$
Sorry , i am not able to understand how you are getting "$int_{-infty}^{+infty}mathbf1_{left(-infty,frac{y-3}2right]}(u)f(u)du$ " from "$int_{-infty}^{frac{y-3}2}f(u)du=$";could you please explain it...
$endgroup$
– Suresh
Jan 8 at 10:22
$begingroup$
Sorry , i am not able to understand how you are getting "$int_{-infty}^{+infty}mathbf1_{left(-infty,frac{y-3}2right]}(u)f(u)du$ " from "$int_{-infty}^{frac{y-3}2}f(u)du=$";could you please explain it...
$endgroup$
– Suresh
Jan 8 at 10:22
1
1
$begingroup$
If $Asubseteqmathbb R$ is measurable then $int_Af(u);du$ is nothing else but a notation for $int_{-infty}^{infty}mathbf1_A(u)f(u)du$. If $A=(a,b]$ then $int_a^bf(u)du$ is a notation for $int_{(a,b]}f(u)du=int_{-infty}^{infty}mathbf1_{(a,b]}(u)f(u)du$. You can take $a=-infty$ here.
$endgroup$
– drhab
Jan 8 at 10:25
$begingroup$
If $Asubseteqmathbb R$ is measurable then $int_Af(u);du$ is nothing else but a notation for $int_{-infty}^{infty}mathbf1_A(u)f(u)du$. If $A=(a,b]$ then $int_a^bf(u)du$ is a notation for $int_{(a,b]}f(u)du=int_{-infty}^{infty}mathbf1_{(a,b]}(u)f(u)du$. You can take $a=-infty$ here.
$endgroup$
– drhab
Jan 8 at 10:25
add a comment |
$begingroup$
$$F_Y(y) = F_Xleft(frac{y-3}{2} right)$$
Now, let's differentiate with respect to $y$ by chain rule
$$frac{d}{dy}F_Y(y) = frac{d}{dy}F_Xleft(frac{y-3}{2} right)=f_Xleft(frac{y-3}{2} right)frac{d}{dy}left(frac{y-3}{2} right)$$
that is $$f_Y(y)=frac12f_Xleft(frac{y-3}{2} right)$$
$endgroup$
add a comment |
$begingroup$
$$F_Y(y) = F_Xleft(frac{y-3}{2} right)$$
Now, let's differentiate with respect to $y$ by chain rule
$$frac{d}{dy}F_Y(y) = frac{d}{dy}F_Xleft(frac{y-3}{2} right)=f_Xleft(frac{y-3}{2} right)frac{d}{dy}left(frac{y-3}{2} right)$$
that is $$f_Y(y)=frac12f_Xleft(frac{y-3}{2} right)$$
$endgroup$
add a comment |
$begingroup$
$$F_Y(y) = F_Xleft(frac{y-3}{2} right)$$
Now, let's differentiate with respect to $y$ by chain rule
$$frac{d}{dy}F_Y(y) = frac{d}{dy}F_Xleft(frac{y-3}{2} right)=f_Xleft(frac{y-3}{2} right)frac{d}{dy}left(frac{y-3}{2} right)$$
that is $$f_Y(y)=frac12f_Xleft(frac{y-3}{2} right)$$
$endgroup$
$$F_Y(y) = F_Xleft(frac{y-3}{2} right)$$
Now, let's differentiate with respect to $y$ by chain rule
$$frac{d}{dy}F_Y(y) = frac{d}{dy}F_Xleft(frac{y-3}{2} right)=f_Xleft(frac{y-3}{2} right)frac{d}{dy}left(frac{y-3}{2} right)$$
that is $$f_Y(y)=frac12f_Xleft(frac{y-3}{2} right)$$
answered Jan 8 at 9:57
Siong Thye GohSiong Thye Goh
104k1468120
104k1468120
add a comment |
add a comment |
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1
$begingroup$
Use the change of variable $yto2y+3$ in the very last integral.
$endgroup$
– Did
Jan 8 at 9:56
$begingroup$
if we put $y to 2y +3 $ , then we will get $f_Y(y)=2 f_X(2y +3)$,now how to proceed?
$endgroup$
– Suresh
Jan 8 at 10:12
$begingroup$
No we do not get that.
$endgroup$
– Did
Jan 8 at 14:30
1
$begingroup$
Wrong change of variable, possibly at least partly due to the horrendous practice of using the same symbol for the bound of the integral and for the variable of integration. Somehow, you are trying to get mistakes...
$endgroup$
– Did
Jan 9 at 6:51
1
$begingroup$
You are doing the change of variable, backwards.
$endgroup$
– Did
Jan 10 at 10:38