Find the exact value of
cos
⁡
A
cosA in simplest radical form.

To calculate the z-statistic, we must first calculate the standard error. Standard error is standard deviation divided by the square root of the sample. In this case, it is equal to 0.178. The z-score is defined the distance from the sample to the population mean in units of standard error. z = (3 – 2.5)/0.178 = 2.81 at this z-score, the probability is 0.9974