forward trailing p/e understand, forward, months, should, price

Walex

hi guys,
does anyone know why justified trailing p/e is (1-b)(1+g)/ (r-g)
while justified forward p/e = (1-b)/(r-g)

I thought that trailing shouldn't have the (1+g) in the numerator while forward should. I don;t understand why we would use (1+g) in the numerator if we're calculating the last twelve months' price and earnings.

Any ideas?