How to find skewness ?

#1
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The method have been used here is two point estimate method (TPEM).There are random variables
from 1 to 24 for a stochastic function. In formula number (1) we should find 2 locations (k=1,2) for each variable. The
,
,
,
are Mean , standard deviation , skewness and weights respectively and m is the last amount of random variable which is 24. The supposed distribution in my problem is zero mean normally distribution. I have read in many textbooks that skewness of normal distribution is equal to zero.We have the amount of mean and standard deviation from a table for each hour(1 to 24) which is
. Now, my problem is why the formula (4 )for skewness has been considered in normal distribution in the paper? and how to calculate
which is represent of expectation value?