Invertible Time Series, MA of Order Infinity 

In the MA(infinity) part, why is (1-0.4B)^-1 = 1+0.4B+(0.4B)^2+...?

(iv) at 12:44

MAKE CLEAR WHEN YOU ARE WRITING ?

For AR Stationarity which is given as 1-0.4z = 0 Where is 1 coming from And where is the contant 0.1