設4階矩陣A=(aij)不可逆，a12的代數余子式A12不等于0，為矩陣A的列向量組
已知離散型隨機變量X的概率分布為
X10203040
P a
(1)求常數a;(2)求X的數學期望EX.
隨機變量X的概率分布Y表示X被3除的余數，則E(Y)=______.
Despitehavingreceivedpositivereviewsbyindustrycritics,salesofthedigitalMP3playerhavebeenquite_______(disappoint).
ExcelEnergyCorporation'snewonlinetrainingprogramhas_______(interact)featuresandforumdiscussionsfortrainees.
