A：$e^x\le 1+x+x^2$

B：$\frac{1}{\sqrt{x+1}}\le 1-\frac{1}{2}x+\frac{1}{4}{x}^2$

C：$\cos x\ge 1-\frac{1}{2}{x}^2$

D：$\ln (1+x)\ge x-\frac{1}{8}{x}^2$