Thread: Quick Help with Joint Probability Mass function

Trying to do this question.

Finding K i get 1/25 but I know this is wrong.

MY method P(1,1) + P(1,3) + P(2,3) = 1

Thanks in advance for your help.

Hello,

No, you're all correct !


Note : please next time, put your attachments as .jpg, not .bmp, as it's more painful to open !

Cool thanks! But I thought that P(1,1) = 1/3 and if K= 1/25

Then P(1,1) = 2/25?

Sorry about the pic format I usually change it but forgot this time.

Originally Posted by Niall101

Cool thanks! But I thought that P(1,1) = 1/3 and if K= 1/25

Then P(1,1) = 2/25?

Sorry about the pic format I usually change it but forgot this time.

Why would P(1,1)=1/3 ? Because there are only 3 points ?
This is only correct if we assume that the distribution is uniform.
Which is not necessarily the case.

Yes, P(1,1)=2/25

Cool Thanks so much! Im glad i was on the right track.

Does that same idea hold for when Im getting the marginal function of X

I get 2x^2 / 25 + 10/25 which is getting X over all values of Y where Y can be 1 or 3.

But is it also possible say for x = 1 and y = 50 for example which would still give a value for X even though the joint would give 0?

Perhaps it'll be clearer if you see it written this way :

Let be the domain of X, namely {1,2}
Let be the domain of Y, namely {1,3}

Then for any , we have :


Which exactly gives what you got :

2x^2 / 25 + 10/25

if x is in

If , we still have , but then these values are 0, according to the definition of P(.,.)

Hey, thanks a lot for your help I appreciate it.