从正态总体N(μ, [img=14x17]17e0a7cfac8081f.jpg[/img]),中抽取容量为5的样本,其观测值为1.863.221.464.012.64（1）已知μ=3,求[img=14x17]17e0a7cfac8081f.jpg[/img]的0.95置信区间；（2）如果μ未知，求[img=14x17]17e0a7cfac8081f.jpg[/img]的0.95置信区间．