Calculating dx when there is aspect ratio
 

HI
At the end of Suhaz V . Patankar Numerical heat transfer book there is T-teach computer program that for calculatig first dx has calculating below:

sumx=0.5*expfac**(n-4)+(expfac**(n-3)-1.0)/(expfac-1.0)+0.5
dx=altot/sumx

expfac=expansion factor
n= number of cells in x direction
altot=for example length of pipe

if expansion factor is assuming 1.2 then each dx should greater than previous cell and it's equal to dx*expfac. in that case we have series:
dx=(1+expfac+expfac**2+expfac**3+...+expfac**n)/altot
i don't understand the origin of first the formula with red colour please help me?