設f(x,y)具有一階偏導數,且對任意的(x,y),
EachyearintheUnitedStates,manyblackteenagers_______ofschool,eitherbecausetheycannotkeepuportheyhavetoworktosupporttheirfamily.
Thistest________anumberofmultiplechoicequestions.
Wemust______thattheexperimentiscontrolledasrigidlyaspossible.
已知離散型隨機變量X的概率分布為
X10203040
P0.20.10.5a
(1)求常數a;(2)求X的數學期望EX.