I get $1 spend 90c and save 10c - you get the 90c spend 81c and save 9c - and so on... eventually the $1= 1/(1-a)= 1/0.1 = $10
... got it
As the demand increases so does ecos growth, as if AD>AS there will be growth - because as more is demanded more is produced to meet the demand
and ecos growth is defined as greater goods produced.
If you don't understand this you should drop ecos, seriously