Find the value of tα/2 when n = 18 for the 99% confidence interval for the mean.

We have to find the critical t value for 99% confidence level and sample size of 18.

Degrees of freedom = n - 1 = 17

α = 1 - 0.99 = 0.01
α/2 = 0.01/2 = 0.005

So, from the t table we have to check the value against the column 0.005 and for 17 degrees of freedom.

This value comes out to be 2.898.

So, tα/2 = 2.898