need help with few questions:

1) Find all open intervals on which $\displaystyle f(x)=\frac{x^2}{x^2+4}$ is decreasing.

this is what i did:

a. fount the derivative: $\displaystyle f'(x)=\frac{8x}{(x^2+4)^2}$

b. set it equal to zero: $\displaystyle 8x=0$

c. $\displaystyle -{\infty}<x<0$ decreasing

is that all you do? I'm concern for this problem b/c they say to "Find all open interval".s

2) Consider $\displaystyle f(x)=x^{1/2}$. Find all value(s), c, in the interval [0,1] such that the slope of the tangent line to the graph of f at c is parallel to the secant line through the point (0, f(0)) and (1, f(1)).

$\displaystyle \frac{1}{2x^{1/2}}$ i have no idea what to do next...

3) Explain why $\displaystyle f(x)=\frac{1}{x}$ has a minimum on the interval [1,2] but not on the interval [-1,1].

graph:

is it b/c the line is decreasing at interval [1,2]?

thx in advance