Answer
$\frac{x+2}{2(1+x)^{3/2}}$
Work Step by Step
1. $\sqrt {1+x}-x\cdot\frac{1}{2\sqrt {1+x}}=\frac{2(1+x)-x}{2\sqrt {1+x}}=\frac{x+2}{2\sqrt {1+x}}$
2. $\frac{\sqrt {1+x}-x\cdot\frac{1}{2\sqrt {1+x}}}{1+x}=\frac{x+2}{2(1+x)\sqrt {1+x}}=\frac{x+2}{2(1+x)^{3/2}}$