Let f(x)=x^3+x^2-1. Prove that for any positive real numbers a, b, c, d satisfying a+b+c+d>\dfrac{1}{a}+\dfrac{1}{b}+\dfrac{1}{c}+\dfrac{1}{d}, at least one of the numbers af(b), bf(c), cf(d), df(a) is different from 1.
Proposed by Adrian Andreescu, Dallas, Texas, USA
Solution:
Assume by contradiction that af(b)=bf(c)=cf(d)=df(a)=1. The given inequality becomes
\begin{equation}\label{first-ineq} a+b+c+d>f(a)+f(b)+f(c)+f(d). \end{equation}
Observe that f(x) is increasing on (0,+\infty) and f(x)<x if and only if x<1. Moreover, f is convex on (0,+\infty), so by inequality \eqref{first-ineq} and Jensen's Inequality, we have \dfrac{a+b+c+d}{4}>\dfrac{f(a)+f(b)+f(c)+f(d)}{4} \geq f\left(\dfrac{a+b+c+d}{4}\right). It follows that \dfrac{a+b+c+d}{4}<1, i.e. a+b+c+d<4 and from \eqref{first-ineq}, we get
f(a)+f(b)+f(c)+f(d)<4. On the other hand, by the AM-GM Inequality, we have \begin{array}{lll} af(a)+bf(b)+cf(c)+df(d) & \geq & 4 \\ bf(a)+cf(b)+df(c)+af(d) & \geq & 4 \\ cf(a)+df(b)+af(c)+bf(d) & \geq & 4 \\ df(a)+af(b)+bf(c)+cf(d) & \geq & 4. \end{array} Adding these four inequalities, we get (a+b+c+d)(f(a)+f(b)+f(c)+f(d))\geq 16, i.e. f(a)+f(b)+f(c)+f(d) \geq \dfrac{16}{a+b+c+d} > 4, contradiction.
The conclusion follows.
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