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基本常数:1/4πε_0=8.99×10~9Vmc~(-1) 题1:在图一所示的电路中R_1,R_2……R_8是有限电阻器,电流计G与电阻R_8串联后接在B、F两端之间。如果定义α、β如下: α=R_1/R_6,β=(R_2+R_3)/(R_4+R_5),求证当R_5=0时,没有电流流过电流计的条件是α=β。再定义λ=R_4/(R_4+R_5), μ=R_5/R_7, 相似文献
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封屹 《湖南师范大学教育科学学报》2000,(5)
研究具有正负系数线性中立型微分方程 (x(t) -p(t)x(t-τ) )′+Q(t)x(t -σ) -R(t)x(t-δ) =0 ,t≥t0 ,其中P(t) ,Q(t) ,R(t) ∈C([T0 ,+∞ ) ,(0 ,+∞ ) ,τ>0 ,σ>δ≥ 0 ,Q(t) =Q(t) -R(t-σ+δ) >0 ( 0 ) ,获得了保证方程每一解振动的新的充分条件 相似文献
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《中学生数理化(高中版)》2019,(9)
<正>例题如图1所示,滑动变阻器(电阻全值为R_0)与电源(电动势E,忽略内阻)组成分压电路,向负载R供电。输出电压U_R=■整理得U_R=(RR_x)/(R_0R+R_0R_r-R_x~2)E,令k=R/(R_0),x=(R_x)/(R_0),可得U_R=(kx)/(k+x-x~2)E= 相似文献
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若一元二次不等式ax2+bx+c≥0恒成立,且a>0,则b2-4ac≤0.由它易得推广1:若(x-k1)2+(x-k2)2+…+(x-kn)2≥0,则(k1+k2+…+kn)2≤n(k21+k22+…+k2n),当且仅当k1=k2=…=kn时,取等号.证明:略. 相似文献
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题目:当k为何值时,方程(k2-1)x2+2(k+1)x+1=0有实数根?四位同学采取了如下四种不同的解法。甲的解法:∵△=[2(k+1)]2-4(k2-1)=8k+8.∴当8k+8>0,即k>-1时,方程有实数根。乙的解法:∵△=8k+8,∴当8k+8≥0,即k≥-1时,方程有实数根。丙的解法:∵△=8k+8,依题意有:k2-1≠08k+8≥0解之得:k≠±1,k≥-1∴当k>-1且k≠1时,方程有实数根。丁的解法:分别讨论k2-1≠0与k2-1=0两种情:(1)设k2-1≠0,依题意有k2-1≠08k+8≥0解得:k≠±1,k≥-1∴当k>-1且k≠1时,方程有两个实数根;(2)当k=1时,原方程为4x+1=0,有一个实数根;(3)当k=-1时,原方程为0·x+1=0,方程… 相似文献
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提起“b2-4ac”,同学们立即会想到它与一元二次方程ax2+bx+c=0(a≠0)有着密切关系.但笔者通过对近几年国内外数学竞赛题的研究发现它在一元二次方程以外也有应用.首先提出:命题当a+b+c=0时,则有b2-4ac≥0,即b2≥4ac.证明由a+b+c=0得b=-(a+c),所以b2-4ac=[-(a+c)]2-4ac 相似文献
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《数理化学习(初中版)》2006,(8)
比较两个二次根式大小是二次根式运算中经常遇到一种类型题.有的比较简单,有的可能就无从下手,所以就此谈一谈几种方法.一、因式内移原理:若a≥b≥0时,则a≥b.例1比较23和32的大小.解:23=12,32=18.因为12<18,所以23<32.对于-23和-32大小比较同样适用.二、平方法原理:a≥0,b≥0且a2≥b2,则a≥b.例2比较2+7与3+6的大小.解:(2+7)2=(2)2+(7)2+2·2·7=9+214(3+6)2=(3)2+(6)2+2·3·6=9+218因为2+7>0,3+6>0,所以2+7<3+6.三、做差法原理:a-b≥0,则a≥b.例3比较2+33与4-33的大小.解:(2+33)-(4-33)=2+33-4+33=63-2=108-4因为108>4,所以(2+33)-(4-33)… 相似文献
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《海外英语》2007,(5):10-11
Many college freshmen arrive woefully unprepared to do college work, and as disadvantaged populations continue to grow, the share of the American work force that has made it through college is expected to plummet. Many experts blame that educational failure not just on high schools but also on colleges. School & College, a special report by The Chronicle, looks at efforts to fix the system. What reforms would better prepare students for college? What should schools and colleges be doing differently? How should state and federal officials help? 相似文献
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罗艳萍 《读与写:教育教学刊》2008,5(6)
The communication of people partially is the communication of cultures. Culture has a direct effect on international commercial activities in all aspects. Different conceptions about time, space, equality, law and the like, lead people to deal with things in different ways. So to know cultures of the counterpart is to facil-itate our enterprises so as to have a smooth and successful communication in commercial activity. 相似文献
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《海外英语》2007,(4):36
There are numbers of crossroads on our long and unpredictable life journey where we totally have no idea about which direction to choose. No matter what our decision is, we should not turn back, but face the music and go ahead instead. I am this kind of girl who always does try without regretting, one example is how I dealt with my love. 相似文献
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Migration occurs behind a variety of reasons and has a great effect on the whole world. People may migrate in order to improve their economic situation, or in order to escape civil strife, persecution, and environmental disasters. The impact of migration is complex, bringing both benefits anddisadvantages. This paper briefly talks about the causes of migration, the allocation of benefits, and the ways in which individual countries and the international community deal with this important subject. 相似文献
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Sister Carrie is one of the most controversial characters in American literature.Thought as a "fallen woman" firstly,she was defined as a "new woman" by some critics later. However, by digging into the motivaton behind the whole process of Carrie's "success", the relationship between Carrie and her creator (the author), the social conditions of then American, it can be found that Carrie has never been free-standing on her thought and she has never found her real-sdf even after becoming a famous actress. In a society dominated by mass consumerism Carrie is only an adherent of her own desires. She also is a representative of all those country girls flooded into cities, a symbol and a sacrifice of the urbanization of America in a time countryside was overcome by cities. 相似文献
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RUZBEHANI Mohsen 《重庆大学学报(英文版)》2004,3(1):20-25
1.IntroductionOne-cyclecontrolmethod,whichwasproposedaboutonedecadeago[1],hasbecomeanattractivemethodinspecialfieldssuchaspowerfactorcorrection[2-6],switchingamplifiers[7,8],etc.Themainideaofthiscontrollerisbasedonintegrationofdiodevoltageinone-cycleandforcesittobeexactlyequaltothereferencevalue.Themainadvantageofthiscontrollerisitsrealtimeabilitytorejectthevariationofinputvoltage[1].Despitethisgreatability,ithasnogoodperformancesinrejectingofloaddisturbanceandfollowingreferencecommands.Espec… 相似文献
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Richard Denner 《海外英语》2007,(4):14-15
Rosco and I wait for the fishermen to return.I sit at a wooden bench near the store at Mt.Baker Resort and watch the clouds change shape. Rosco has my belt around his neck and an eight foot tow chain hooked to a tree. Dogs must be on a leash. Ducks and rabbits are loose. 相似文献