Find the value of (f o g)' at the given value of x. f(u)=u^5+1, u=g(x)=sqrt x, x = 1 So I found the derivate of f(u) & of u. f'(u)= 5u^4 & u'= 1/(2sqrt x) Then I plugged 1 in for x in u' & got 1/2. What I don't understand is why I can't just then plug 1/2 in for u & solve. Another example f(u)= 2u/(u^2+1), u=g(x)= 10x^2+x+1, x=0 Here I did the same, found the derivative of f'(u) & u'. f'(u) = -2u^2+2/(u^2+1)^2 & u'=20x+1 Then I plugged in zero for u' & got 1. If you then plug 1 into f'(u), you get 0. Which matches the answer in the back of the book, but the first one does not using the same technique. Somewhere I think my knowledge of the concept is incomplete. Any help would be welcome.