证明:对任意 $x\in\mathbb R^{\ast}$,有\[\max\{0,\ln |x|\}\geqslant \dfrac{\sqrt 5-1}{2\sqrt 5}\ln |x|+\dfrac1{2\sqrt 5}\ln |x^2-1|+\dfrac 12\ln\dfrac{\sqrt 5+1}2,\]且等号成立的充要条件是 $x=\pm\dfrac{\sqrt 5+1}2$ 或 $x=\pm \dfrac{\sqrt 5-1}2$.
解析
根据对称性,只需要考虑 $x>0$ 且 $x\ne 1$ 的情形,记 $\dfrac{\sqrt 5+1}2=a$,则 $\dfrac{\sqrt 5-1}2=\dfrac 1a$.
情形一 $x\in (0,1)$.此时命题即\[\forall x\in (0,1),\dfrac{\sqrt 5-1}{2\sqrt 5}\ln x+\dfrac1{2\sqrt 5}\ln (1-x^2)+\dfrac 12\ln\dfrac{\sqrt 5+1}2\leqslant 0,\]且等号取得的条件是 $x=\dfrac{\sqrt 5-1}2$.该不等式即\[\sqrt 5\ln x+\ln \left(\dfrac 1x-x\right)+\sqrt 5\ln\dfrac{\sqrt 5+1}2\leqslant 0,\]也即\[x^{\sqrt 5-1}-x^{\sqrt 5+1}\leqslant \left(\dfrac{\sqrt 5+1}2\right)^{-\sqrt 5},\]记左侧为函数 $f(x)$,则其导函数\[f'(x)=\left(\sqrt 5-1\right)x^{\sqrt 5-2}-\left(\sqrt 5+1\right)x^{\sqrt 5}=\left(\sqrt 5+1\right)x^{\sqrt 5-2}\left(\left(\dfrac{\sqrt 5-1}2\right)^2-x^2\right),\]因此等 $x=\dfrac{\sqrt 5-1}2$ 时函数 $f(x)$ 取得极大值,也为最大值 $\left(\dfrac{\sqrt 5+1}2\right)^{-\sqrt 5}$,命题得证.
情形二 $x\in (1,+\infty)$.此时命题即\[\forall x\in (1,+\infty),\dfrac{\sqrt 5-1}{2\sqrt 5}\ln x+\dfrac1{2\sqrt 5}\ln(x^2-1)+\dfrac 12\ln\dfrac{\sqrt 5+1}2\leqslant \ln x,\]且等号取得的条件是 $x=\dfrac{\sqrt 5+1}2$.该不等式即\[-\sqrt 5\ln x+\ln\left(x-\dfrac 1x\right)\leqslant \left(\dfrac{\sqrt 5 +1}2\right)^{-\sqrt 5},\]左侧即 $f\left(\dfrac 1x\right)$,因此其最大值当 $\dfrac 1x=\dfrac{\sqrt 5-1}2$,即 $x=\dfrac{\sqrt 5+1}2$ 时取得,最大值为 $\left(\dfrac{\sqrt 5+1}2\right)^{-\sqrt 5}$,命题得证.
综上所述,原命题得证.