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The position vector of a point P(x,y) in two dimensions is xi yj We will often denote this important vector by r See Diagram 2 (In three dimensions the position vector is r = xiyj zk) x y 0 i j Diagram 1 x y 0 xi yj P(x,y) r Diagram 2 Section 1 Introduction (Vectors) 4 The vector differential operator ∇, called "del" or "nabla", is defined in three dimensions to be ∇P{X Y ≤ 3} 3 P{XY = 0} 4 P{X = 3} As before, the mass function has two basic properties • f X,Y (x,y) ≥ 0 for all x and y 1 • P x,y f X,Y (x,y) = 1 The distribution of an individual random variable is call the marginal distribution The marginal mass function for X is found by summing over the appropriate column and the marginal mass function for Y can be found be summing overLet X and Y be two discrete rv's with a joint pmf fX;Y(x;y) = P(X = x;Y = y) Remember that the distributions (or the pmf's) fX(x) = P(X = x) of X and fY(y) = P(Y = y) of Y are called the marginal distributions of the pare (X;Y)...