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ObliviousSage
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Two "Sum of Random Variables" Problems
Problem A:
Consider two independent uniform random variables on [0,1]. Compute the probability density function for Y = X1 + 2X2.
Problem B:
Edit: never mind, solved this one
fY(y) = F'Y(y)
FY(y) = double integral (X1+2X2 <= y) fX1X2(x1,x2)
fX1X2(x1,x2) = fX1(x1)*fX2(x2) since they're independent
for uniform RVs on [a,b], fX(x) = 1/(b-a) = 1 in this case
We worked the Y=X1+X2 version of this in class, so I feel like I at least have some idea where to start. Since X only exists inside [0,1], I know I have to break that implicit integral down into several ranges. It looks like I need to break it down into 0<y<1, 1<y<2, and 2<y<3. I can reduce the 0<y<1 case to integral(0 to y/2) dx2 * integral(0 to y-2x2) dx1.
My problem is the other two ranges. I know I can express the 2<y<3 range as 1 - the triangle above the y=X1+2X2 line, I'm just not sure how to describe that triangle. As for the 1<y<2 range, that's not even a triangle so I'm not sure how to explicitly set the limits of integration; don't I need to break it up into 2 different integrals? We haven't worked any examples like that in class, and it's been over ten years since I took calculus, so...
Homework Statement
Problem A:
Consider two independent uniform random variables on [0,1]. Compute the probability density function for Y = X1 + 2X2.
Problem B:
Edit: never mind, solved this one
Homework Equations
fY(y) = F'Y(y)
FY(y) = double integral (X1+2X2 <= y) fX1X2(x1,x2)
fX1X2(x1,x2) = fX1(x1)*fX2(x2) since they're independent
for uniform RVs on [a,b], fX(x) = 1/(b-a) = 1 in this case
The Attempt at a Solution
We worked the Y=X1+X2 version of this in class, so I feel like I at least have some idea where to start. Since X only exists inside [0,1], I know I have to break that implicit integral down into several ranges. It looks like I need to break it down into 0<y<1, 1<y<2, and 2<y<3. I can reduce the 0<y<1 case to integral(0 to y/2) dx2 * integral(0 to y-2x2) dx1.
My problem is the other two ranges. I know I can express the 2<y<3 range as 1 - the triangle above the y=X1+2X2 line, I'm just not sure how to describe that triangle. As for the 1<y<2 range, that's not even a triangle so I'm not sure how to explicitly set the limits of integration; don't I need to break it up into 2 different integrals? We haven't worked any examples like that in class, and it's been over ten years since I took calculus, so...
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