Question 384376
If xi were the datas, and x the average, x=(x1+...+xn)/n
 
the deviation were then dxi=xi-x
 
the sum of deviations : dx1+...+dxn=x1+...+xn-nx but nx=x1+...+xn

hence dx1+...dxn=x1+...xn-x1-...-xn=0