Let X , Y denote , no of red , white balls chosen resp.
find the Joint pdf of X and Y
I dont understand why ans like this , where the 1/36 comes from ?
1/36 (summation x=1 to 2 )(summation y=1 to 3-x)xy
why x is from 1 to 2
y from 1 to 3-x, not 0 to 3-x ??????????
Another version is f)1,1_+f(1,2)+f(2,1)
=1/36+2/36+2/36
Actually, how to count them?
更新1:
果真如此, 那相應的題目是 f)x y ))=cxy ,x y =1,2,3 find c and P(X+Y less than 3 ) 我可看得明 但本題的答案計了 f(0 0 )= (3 0 )(4 0 )(5 0)/(12 3 ) =10/220 (n r ) is nCr f(1,2)=18/220 P(x=0)=f(0,0)+f(0,1)+f(0, 2 )+f(0,3) P(y=2)=f 0 2 +f 1 2 怎再做落圠?