Solve $\sqrt{\dfrac{1-x}{1+x}} = \dfrac{1}{2}$. Then $x =$

Squaring gives $\dfrac{1-x}{1+x} = \dfrac{1}{4}$, so $4(1-x) = 1+x$. This yields $5x = 3$, hence $x = \dfrac{3}{5}$.