let function
$$f_{n}(x)=\left(1+x+\dfrac{1}{2!}x^2+\cdots+\dfrac{1}{n!}x^n\right)\left(\dfrac{x^2}{x+2}-e^{-x}+1\right)e^{-x},x\ge 0,n\in N^{+}$$
if $\lambda_{1},\lambda_{2},\mu_{1},\mu_{2}$ is postive numbers,and such $\mu_{1}+\mu_{2}=1$
Question: following which is bigger: $$f_{n}\left[(\lambda_{1}\mu_{1}+\lambda_{2}\mu_{2})\left(\dfrac{\mu_{1}}{\lambda_{1}}+\dfrac{\mu_{2}}{\lambda_{2}}\right)\right], f_{n}\left[\dfrac{(\lambda_{1}+\lambda_{2})^2}{4\lambda_{1}\lambda_{2}}\right]$$
My try: since $$e^x=1+x+\dfrac{1}{2!}x^2+\cdots+\dfrac{1}{n!}x^n+\cdots+$$ But this problem is
$$1+x+\dfrac{1}{2!}x^2+\cdots+\dfrac{1}{n!}x^n$$ so How prove it?Thank you
This problem is from http://tieba.baidu.com/p/2682214392