1+5+9 +... is a sequens av positiv integers with start=1 & step size=4
last number=1+stepsize*(n-1)
averaej=(start number +last number)/n
sum=n*averaej/2
for stepsize=4, last=1+4*(n-1)
averaej=2+4*(n-1)
sum=n*[1+2*(n-1)] or n*(2n-1)
zampel...n=5: sum=5*9=45