tabulated integral是什么意思

如何计算tabulated value required for a 95% confidence interval
a random sample of size 30 is obtained from a normally distributed population.the mean of this sample is 38.1 and the standard deviation of this sample is 8.5.
since sigma,the population standard deviation is unknown,we shall have to calculate a confidence interval using a critical value of the t distribution with n-1 degrees of ***.
1.what is the estimated standard error of the mean for samples of this size?(2 dp)
2.what are the degrees of *** for this (integer)
3.what is the tabulated value required for a 95% confidence interval?
(3 dp)
based on this sample,the 95% confidence interval for m is given by:
(lower limit,upper limit) = (_._ ,_._)
4.lower limit (2 dp)
5.upper limit
应该是很基础的题就是第3问不太明白怎么算,能简单介绍下基本定义么