例2设连续型随机变量X的密度函数f(x)为 2xx∈[0,1 f(x) 0 x≠[0,1 求:Var(X) ar(X)=E(X2)-[E(X)2 ∫×/(.(o 〔2-.223 18求:Var(X) 解: 例 2 设连续型随机变量X的密度函数f(x)为:      = 0 [0,1] 2 [0,1] ( ) x x x f x 18 1 3 2 2 1 2 2 ( ) ( ) ( ) ( ) [ ( )] 2 2 1 0 1 0 2 2 2 2 2  =      = −     =  −      = − = −      −  − x xdx x xdx x f x dx x f x dx Var X E X E X