设x1，x2，…，xn1与y1，y2，…，yn2分别是来自总体N(μ1，σ2)与(μ2，σ2)的两个样本，它们相互独立，且<img src="http://www.weimei168.com/uploads/202208/161223534272331.png" width="17" height="37" />，<img src="http://www.weimei168.com/uploads/202208/161223534730791.png" width="19" height="40" />分别为两个样本的样本均值，则<img src="http://www.weimei168.com/uploads/202208/16122353547711.png" width="17" height="37" />-<img src="http://www.weimei168.com/uploads/202208/161223535926391.png" width="19" height="40" />所服从的分布为（）