设随机变量<img src="/lqwvjeImg/a0/a0e3f3124b2115bd8f51e19ae344bc91.png" style="width:45px;height:21px;" >服从二维正态分布,其联合概率密度函数为:<img src="/lqwvjeImg/78/78ece40259e1ffab1f5c1ab88512697b.png" style="font-size: 14px;" >,则随机变量<img src="/lqwvjeImg/4d/4da57b561578dc3f0d1e8ac698961f61.png" style="width: 96px; height: 25px;" >的数学期望<img src="/lqwvjeImg/15/15f421fed40c30bad750e82575212376.png" style="width: 25px; height: 17px;" >为( )。<br>·<img src="/lqwvjeImg/58/58ab4ec21884f1d52547b6d76e713c24.png"style="width:29px;height:46px;"><br>·<img src="/lqwvjeImg/ee/ee1533f16c6e8c827a3037647f8f161d.png"style="width:17px;height:42px;"><br>·2<br>·<img src="/lqwvjeImg/cf/cff56fbe44271ef15e1cfd48cf7687a0.png"style="width:15px;height:15px;">