Transcription of Examples: Joint Densities and Joint Mass Functions
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AMS 311 Joe MitchellExamples: Joint Densities and Joint Mass FunctionsExample 1:XandYare jointly continuous with Joint pdff(x, y) = cx2+xy3if 0 x 1, 0 y 20,otherwise.(a). Findc. (b). FindP(X+Y 1). (c). Find marginal pdf s ofXand ofY. (d). AreXandYindependent (justify!). (e). FindE(eXcosY). (f). Findcov(X, Y).We start (as always!) by drawing the support set. (See below, left.)21211y=1 xxyxysupport setBlue: subsetof support setwith y>1 x(a). We findcby setting1 =Z Z f(x, y)dydx=Z10Z20(cx2+xy3)dydx=2c3+13,soc= 1.
the probability, we double integrate the joint density over this subset of the support set: P(X +Y ≤ 1) = Z 1 0 Z 1−x 0 4xydydx = 1 6 (b). Refer to the figure (lower left and lower right). To compute the cdf of Z = X + Y, we use the definition of cdf, evaluating each case by …
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