2005年湖北省数学高考试题文科21题疑点解析

本人在分析研究湖北省今年数学高考题时,发现文科21题的解答(网上、报上公布的)存在可疑之处.原题:某会议室用5盏灯照明,每盏灯各使用灯泡一只,且型号相同.假定每盏灯能否正常照明只与灯泡的寿命有关,该型号的灯泡寿命为1年以上的概率为p1,寿命为2年以上的概率为p2.从使用之日起每满1年进行一次灯泡更换工作,只更换已坏的灯泡,平时不换.(Ⅰ)在第一次灯泡更换工作中,求不需要换灯泡的概率和更换2只灯泡的概率;(Ⅱ)在第二次灯泡更换工作中,对其中的某一盏灯来说,求该盏灯需要更换灯泡的概率;(Ⅲ)当p1=0.8,p2=0.3时,求在第二次灯泡更换工作,至…     

2005年湖北省数学高考试题文科21题疑点解析

本人在分析研究湖北省今年数学高考题时，发现文科21题的解答（网上、报上公布的）存在可疑之处．     

Spintronic terahertz emitter with integrated electromagnetic control

Spintronic thin films are considered as one of the promising terahertz(THz) source candidates, owing to their high performance and low cost. Much effort has been made to achieve spintronic THz sources with broadband and high conversion efficiency. However, the development of spintronic THz emitters with good compatibility, low cost, and miniaturized technology still faces many challenges. Therefore, it is urgent to extend commercial and portable spintronic THz emitters to satisfy many practical ...     

Theoretical calculations of hardness and metallicity for multibond hexagonal 5d transition metal diborides with ReB_2 structure

The hardness, electronic, and elastic properties of 5d transition metal diborides with ReB_2 structure are studied theoretically by using the first principles calculations. The calculated results are in good agreement with the previous experimental and theoretical results. Empirical formulas for estimating the hardness and partial number of effective free electrons for each bond in multibond compounds with metallicity are presented. Based on the formulas, IrB_2 has the largest hardness of 21.8 GPa, followed by OsB_2(21.0 GPa) and ReB_2(19.7 GPa), indicating that they are good candidates as hard materials.