Solutions of Y cOs [G ''(x)] = Arcsin [ f '(x) ]

This post is on one of the wierd thing you'll see if you live on campus: Today was another normal tuesday. Woke up way before for my 8am lecture *fast forward o 3pm* YAY! end of the day! So, i went back to PGP. Well, after putting my stuff down, the time was about 5+. I went to the toilet and guess what i saw: My dear clustermate was just brushing his teeth!! gosh~
How would U greet him? Goodmorning? or Goodafternoon?