$F(b) = \dfrac{b - b^2}{1 + b^2}$
$F(0) = \dfrac{0 - 0^2}{1 + 0^2} = 0$
$F(1) = \dfrac{1 - 1^2}{1 + 1^2} = 0$
$F(\frac{1}{2}) = \dfrac{\frac{1}{2} - (\frac{1}{2})^2}{1 + (\frac{1}{2})^2} = \dfrac{\frac{1}{2} - \frac{1}{4}}{1 + \frac{1}{4}} = \dfrac{\frac{1}{4}}{\frac{5}{4}} = \dfrac{1}{5}$
$F(\tan x) = \dfrac{\tan x - \tan^2 x}{1 + \tan^2 x}$
$= \dfrac{\tan x (1 - \tan x)}{\sec^2 x}$
$= \cos^2 x \left( \dfrac{\sin x}{\cos x} \right) \left( 1 - \dfrac{\sin x}{\cos x} \right)$
$= \sin x (\cos x - \sin x)$