in reading 36 of EOC practice’s question 12B, it asks
What trailing P/E, P/B, and P/S multiples would be justified in light of the required rate of return in Part A and current values of the dividend payout ratio, ROE, and profit margin? then the calculation used (1-b)(1+g)/(r-g). if we are just talking about trailing PE, why do we apply （1+g) on the equation?
This is one of the most popular questions around this forum.
This one is the latest I remember. http://www.analystforum.com/forums/cfa-forums/cfa-level-ii-forum/91348889
I wrote an article on justified ratios that may be of some help here: http://financialexamhelp123.com/justified-ratios-price-multiples/
Check the derivation and you will find out 
The leading P/E is lower than trailing P/E:
Leading PE= (1-b)/(r-g)
Remember that both PE use Po and the only thing that varies is E, so when E increases, the PE decreases. That’s why leading PE is lower than trailing PE.