@Kitonum Well damn..I'm not sure how I missed that. Thank you so much!

@Kitonum Everything went well until the end.. Does this method not give a value for H? It just gave me an equation.

@Mac Dude 

OH thank you so much!! Now, I'm having the same problem with a simialr question.

g := (1/8)*x*sin(13/x)+arcsin(5*x^2/(8*x^2+1))

G := unapply(g, x)

H(x) :=G(G(x))

Similar to the last question, I need to find the value of H'(pi/2). I tried this, but it didn't work;

simplify(eval(diff(H(x), x), x = (1/2)*Pi))

Will it not work for this type of question? Because this method worked for my previously asked question.

Thanks :)