Maths Olympiad Prep

Track / Stage 3 / 245 of 260 #245 of 1964

Problem 245

AMC 10/12, early questions
Algebra Difficulty 3.9 Find the answer

Given the function f(x)={x+1x1,(x>0)x3+1,(x0)f(x) = \begin{cases} x+ \frac {1}{x}-1, & (x>0) \\ -x^{3}+1, & (x\leq0)\end{cases},
(I) Find the minimum value of the function f(x)f(x);
(II) Let mRm\in\mathbb{R}, proposition pp: The inequality f(x)m2+2m2f(x) \geq m^2+2m-2 holds for any xRx\in\mathbb{R}; proposition qq: The exponential function y=(m21)xy=(m^2-1)^x is increasing. If "p or q" is true, and "p and q" is false, find the range of the real number mm.

A number or a short expression. Spacing, $ signs and \frac vs / are all fine.

Official solution

Solution:
(I) Given f(x)={x+1x1,(x>0)x3+1,(x0)f(x) = \begin{cases} x+ \frac {1}{x}-1, & (x>0) \\ -x^{3}+1, & (x\leq0)\end{cases},
For x>0x>0, according to the graph and properties of the root function,
we have: when x=1x=1, the function reaches its minimum value of 1,
For x0x\leq0, the function is decreasing,
when x=0x=0, the function reaches its minimum value of 1,
Overall, the minimum value of the function f(x)f(x) is 1\boxed{1}.
(II) From (I), we have m2+2m21m^2+2m-2\leq1 holds for any xRx\in\mathbb{R}
which means m2+2m30m^2+2m-3\leq0, solving this gives 3m1-3\leq m\leq1,
Therefore, for proposition pp: 3m1-3\leq m\leq1.
For proposition qq: The exponential function y=(m21)xy=(m^2-1)^x is increasing,
thus m21>1m^2-1>1,
Therefore, for proposition qq: m2m\sqrt{2}.
If "p or q" is true, and "p and q" is false,
then pp, qq are true and false respectively,
Considering two cases:
If pp is true and qq is false, then {3m12m2\begin{cases} -3\leq m\leq1 \\ -\sqrt{2}\leq m\leq \sqrt{2}\end{cases}, solving this gives 2m1-\sqrt{2}\leq m\leq1.
If pp is false and qq is true, then {m1m2\begin{cases} m1 \\ m\sqrt{2}\end{cases}, solving this gives m2m\sqrt{2}.
Therefore, the range of the real number mm is m2m\sqrt{2}, which can be summarized as m2\boxed{m\sqrt{2}}.

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic, difficulty and ordering added by this site.