The Z-distribution is a normal distribution with mean zero and standard deviation 1; its graph is shown here. Almost all (about 99.7%) of its values lie between –3 and +3 according to the Empirical Rule. Values on the Z-distribution are called z-values, z-scores, or standard scores
Tan (90 - A) = sin (90 - A) / cos (90 - A) = (sin 90 cos A - sin A cos 90)/(cos 90 cos A + sin 90 sin A) = cos A / sin A = 1 / (sin A / cos A) = 1/tan A