何灯,李云杰
(福清第三中学,福建福清350315)
双曲函数的Cusa-Huygens型不等式的推广与改进
何灯,李云杰
(福清第三中学,福建福清350315)
本文将双曲函数的Cusa-Huygens型不等式作了含参推广和改进,由此建立的不等式优于现有的诸多结果,文末导出一条涉及算术平均、几何平均、对数平均的不等式链.
双曲函数;Cusa-Huygens型不等式;Seiffert平均;不等式
文献[1-2]建立了著名的Cusa-Huygens不等式,文献[3]给出了Cusa-Huygens不等式的双曲函数形式.针对文献[3]所建立的不等式,J.Sándor、朱灵、杨镇杭、吴善和、陈超平等不等式专家做了大量的研究,现有诸多结果[4-17].本文在现有研究的基础上,建立了shx/x的更强的含参上下界形式,将已有的研究结果做了更进一步的推广和改进,由此得到了涉及算术平均、几何平均、对数平均的一条不等式链.
Cusa-Huygens不等式[1-2]:设,则有.
双曲型Cusa-Huygens不等式[3]:设x∈(0,+∞),则有
朱灵[7]将式(1)推广为:设x>0,,则有.
E.Neuman与J.Sándor则改进式(1)为:设x>0,则.
成立当且仅当q≥3.
朱灵[15]将式(2)推广为:设x>0,p>1或p≤8/15,则当且仅当q≥3(1-p).特别地,令p=1/2,q=3/2,可得
杨镇杭[11]证得式(3)的如下含参推广:
结论1设p,x>0,双边不等式
结论2设x>0,则
综合上述结论,可得不等式链
关于上述不等式链的研究,可参阅文献[17].
引理1设t∈(0,0.88),n∈N*,n≥4,则f(n)=t2n(2n+1)关于n单调递减.
证明f(x)=t2x(2x+1)(x∈R,x≥4),可求
则f(x)=t2x(2x+1)关于x在[4,+∞)上单调递减,从而当n≥4,f(n)=t2n(2n+1)关于n单调递减.
引理2设an=180p4-4p2n(20p2-3)(2n+1)-22np2n(3-5p2)(2n+1),n∈N*,n≥3,,则当p=p1时an≥0,当p∈[1/2,p2]时an≤0.
证明当p=p1,可求a3=0.当n≥4,由引理1得
又
当p∈(0,p1],由引理2,当p=p1,F(p,x)≥0,结合引理3,可证.
当p∈[p3,+∞),注意到,据式(5)显然有F(p3,x)≥0,结合引理3,可证.当p∈[1/2,p2],由引理2,F(p,x)≤0,式(6)反向成立.
注意到
要使F(p,x)≥0,必需有
解之,可得p∈(0,p1]∪[p3,+∞).要使F(p,x)≤0,必需有
解之,可得p∈[1/2,p2].
综上,定理1得证.
注2计算得
足见式(6)在x=0附近有较高的逼近精度.
注意到G(p3/2)=G(p3),则可得
定理3设x>0,则
其中花括号上下两个不等式表示其不分强弱(下同).
注意到
两个正数a,b的幂平均定义为[18]
A2,A1,A0分别称为这两个数的平方根平均,算术平均及几何平均.
反双曲正切Seiffert平均(即对数平均)定义为
从而定理1等价于
定理4设a,b>0,a≠b,p∈R,则
定理5设a,b>0,a≠b,则如下不等式链成立
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Extension and Improvement of the Cusa-Huygens Type Inequalities for Hyperbolic Function
HE Deng,LI Yunjie
(Number 3 Middle School,Fuqing350315,Fujian,China)
A parametric extension and improvement on the Cusa-Huygens type inequalities for Hyperbolic function is presented.The establishment of the inequality is superior to the existing results.An inequality chain involving arithmetic mean,geometric mean,logarithmic mean is explored.
hyperbolic function;Cusa-Huygens type Inequalities;Seiffert mean;inequalities
O178
A
1001-4217(2016)04-0049-08
2015-12-21
何灯(1984—),男,福建福清人,学士,全国不等式研究会成员.研究方向:解析不等式及不等式机器证明.E-mail:hedeng123@163.com