设<img src="http://jgxm-question.oss-cn-beijing.aliyuncs.com/psst/files/ec0b671b70664f6f8e054ee6dbb90b6f.png?x-oss-process=image/sharpen,200"/><img src="http://jgxm-question.oss-cn-beijing.aliyuncs.com/psst/files/4133c2d9e5944b3fbf0c4472f2adff14.png?x-oss-process=image/sharpen,200"/>为来自总体<img src="http://jgxm-question.oss-cn-beijing.aliyuncs.com/psst/files/557c71c34d2e41bb9d1aa13a0b533073.png?x-oss-process=image/sharpen,200"/>的简单随机样本，<img src="http://jgxm-question.oss-cn-beijing.aliyuncs.com/psst/files/1f9fdfcf5f0c46ed95a6da30287cbf66.png?x-oss-process=image/sharpen,200"/>为样本均值，已知<img src="http://jgxm-question.oss-cn-beijing.aliyuncs.com/psst/files/46beb72d539e4555a6515b63c1ed671e.png?x-oss-process=image/sharpen,200"/>是<img src="http://jgxm-question.oss-cn-beijing.aliyuncs.com/psst/files/681a223e7ab24560b9ab9672f9f7e9b1.png?x-oss-process=image/sharpen,200"/>的无偏估计，则常数<img src="http://jgxm-question.oss-cn-beijing.aliyuncs.com/psst/files/3d323ebe88fa4d5793acfbcaaeee31f6.png?x-oss-process=image/sharpen,200"/>必为（）