雙重期望值定理 雙重期望値定理Double expectation theorem亦稱重疊期望値定理Iterated expectation theorem全期望値定理Law of total expectation即设XYZ为随机变量g和h为连续函数下列期望和条件期望均存在则 E   X   E   E   X  Y    displaystyle operatorname E Xoperatorname E operatorname E Xmid Y