四年级数学练一练

一、脱式计算。(160-111÷37)×63500-(540+650÷130)25×14÷(451-449)15×(300-192÷4)560÷40+48×12(#18+25×14)×4二、口算。12×2×5=240÷6÷4=20-5=14-81÷9=15-(8-4)×2=15÷9×6=三、填空。1.二十亿五千六百零四十写作(),改写成以“亿”作单位的数是()亿。2.把4.05扩大10倍是(),把90缩小100倍是()。3.在1974、1990、1999、2000、2004这些年份中,闰年有()。4.线段有()个端点,射线有()个端点,直线有80()个端点。四、判断题。1.25×4÷25×4=16。()2.1990年是闰年。()3.30个月=2年6个月。()4.锐角一定比直角小。()5.小数减法的意义…     

巧记9的口诀

你发现了9的乘法口诀的特点吗?9比10少1(也就是9=10-1),因此,9×1=1个10-1=10-1=9。18比20少2(也就是18=20-2),因此9×2=2个10-2=18。这样,我们就可以发现:9乘以几,结果就是“几十减去几”。知道了这个特点,我们就可以很容易记住9的口诀,于是就有:9×1=10-1=9(口诀:一九得九)9×2=20-2=18(口诀:二九十八)9×3=30-3=27(口诀:三九二十七)9×4=40-4=36(口诀:四九三十六)9×5=50-5=45(口诀:五九四十五)9×6=60-6=54(口诀:六九五十四)9×7=70-7=63(口诀:七九六十三)9×8=80-8=72(口诀:八九七十二)9×9=90-9=81(口诀:九九八十一)小朋友,你记住了吗…     

2003年中考数学模拟试题

一、选择题 (将下列各题中惟一正确答案的序号填入题后括号内 ,每小题 3分 ,共 30分 )1.在实数 -π ,- 2 ,9,ｃｏｓ4 5° ,3.14 ,( 2 ) 0中 ,有理数共有 (　　 )(Ａ) 5个　 (Ｂ) 4个　 (Ｃ) 3个　 (Ｄ) 2个2 .人类的遗传物质就是ＤＮＡ ,人类的ＤＮＡ是很长的链 ,最短的 2 2号染色体也长达 30 0 0 0 0 0 0个核苷酸 ,30 0 0 0 0 0 0用科学记数法可表示为 (　　 ) .(Ａ) 0 .3× 10 6　　 (Ｂ) 3× 10 6(Ｃ) 3× 10 7　　 (Ｄ) 3× 10 83.下列计算中正确的是 (　　 )(Ａ) ( 3ｘｙ) 3 =9ｘ3 ｙ3(Ｂ) 2ａ2 · ( - 3ａ3 ) =- 6ａ6(Ｃ) ( -ａ3 ) 2 …     

利尿剂的循环伏安法检测

用电化学分析系统对两种利尿剂进行同时检测后,初步发现循环伏安法对兴奋剂检测的可行性.其中双克的线性范围为1 51×10-5～1 52×10-4mol·dm-3,检测限为1 76×10-6mol·dm-3;速尿的线性范围为1 31×10-4～1 08×10-3mol·dm-3,检测限为1 80×10-6mol·dm-3,同时对循环伏安法分析条件和电化学反应机理进行了初步探讨.     

亚铁氰化钾修饰碳黑微电极测定抗坏血酸

研究了抗坏血酸(AA)在亚铁氰化钾修饰碳黑微电极上的电化学行为。实验结果表明,在pH7．0的磷酸盐中,AA在该电极上的线性范围为2．0×10-6-4．0×10-3mol／L;检出限(3σ)为1．0×10-6mol／L。用这种电极测定抗坏血酸的含量,结果满意。     

绍兴县2003年初三年级数学中考适应性测试题

一、选择题 ( 12× 4分 )1 三个实数 - 3、- 2、0依次从小到大排列的顺序是 (　　 ) .Ａ - 3<- 2 <0　　Ｂ - 2 <- 3<0Ｃ 0 <- 3<- 2　　Ｄ 0 <- 2 <- 32 2 0 0 2年 5月 15日 ,我国发射的海洋Ⅰ号气象卫星 ,进入预定轨道后 ,若绕地球运行的速度为 7.9× 10 3 米 秒 ,则运行 2× 10 2 秒走过的路程是 (用科学记数法表示 ) .Ａ 15.8× 10 5 米　　Ｂ 1.58× 10 5 米Ｃ 0 .158× 10 7米　　Ｄ 1.58× 10 6 米3 下列运算中 ,正确的是 (　　 ) .Ａ ａ2 ·ａ4 =ａ8　　Ｂ 1-ａ -ｂ =- 1ａ-ｂＣ ａ - 1ａ =- -ａＤ (ｔａｎ30°- 13) 0 =14 下列…     

NaIO4-VB2化学发光反应体系测定维生素B2

利用建立的NaIO4-H+-H2O2-VB2化学发光新体系,结合流动注射技术建立起一种测定维生素B2的化学发光新方法.方法的线性范围为1.0×10-7～5.0×10-5g/mLVB2;检出限为2.7×10-8g/mL;相对标准偏差(RSD)为2.3%(Cs=1.0×10-6g/mLVB2,n=11).     

对课本中阿伏伽德罗常数计算方法的商榷

现行高中物理课本在讲到阿伏伽德罗常数时,介绍了如下的计算方法:“知道分子的大小,不难粗略算出阿伏伽德罗常数,例如1摩水的质量是0.018千克,体积是1.8×10~(-5)米~3,每个水分子的直径是4×10~(-10)米,体积约为π/6(4×10~(-10))~3=3×10~(-29)米~3。设想水分子是一个挨一个排列的,不难算出1摩水中所含的水分子数; N_A=1.8×10~(-5)米~3/摩/3×10~(-29)米~3=6×10~(23)摩~(-1)。这种算法可用下式表示: 阿伏伽德罗常数=1摩水的体积/每个分子(球形)的体积. 这种算法存在明显的问题,把球形分子的体积当成了分子所占空间的体积。如果把分子看作球形,即使是     

暑假数学自测题

一、选择题(每小题2分,共20分) 1.计算(3-1)2,结果等于(　　). 　　　　　　　　　　　　　　　　　　(A)3　　　　(B)6　　　　(C)13　　　　(D)19 2.1纳米=0.000000001米,则27纳米用科学记数法表示为(　　). (A)2.7×10-8米　　　　(B)2.7×10-9米 (C)2.7×10-10米　　　　(D)2.7×109米 3.下列计算中,正确的是(　　). (A)x2+x2=x4　　　　(B)x·x4=x4 (C)x6÷x2=x4　　　　(D)(xy)4=xy4 4.一个多项式减去a2-b2等于a2+b2,则这个多项式是 (　　). (A)2b2　　　　(B)-2b2　　　　(C)2a2　　　　(D)-2a2 5.如图1,a…     

水的电离相关问题类析

水是一种极弱的电解质,在常温下能电离出极少量的H+和OH-,存在着电离平衡,而在解答高考化学试卷中的有关问题时,这个常被忽略的平衡经常起着至关重要的作用,因此必须引起考生足够的重视,应掌握相关的知识和规律·1·KW=c(H+)c(OH-)·由于水中c(H+)=c(OH-),纯水总是中性的·例1某温度时测得纯水中c(H+)=2·4×10-7mol/L,则c(OH-)为()(A)2·4×10-7mol/L(B)0·1×10-7mol/L(C)(1·0×10-14)/(2·4×10-7)mol/L(D)无法确定解析:c(H+)与c(OH-)的乘积等于1·0×10-14就是相当多的学生的思维定势,由此出发,不费力就可找出选项(C),同时…     

卡思卡特城堡

It is worthy of noting that, whilst Crookston Castle witnessed the earlier and happier portion of Mary's variegated life,     

不同的吃喝玩乐

一、吃和喝吃苹果 eat an apple, 吃药 take medicine,吃糖 have some sweets,吃饭 have one's meals,吃馆子 dine out,吃惊 be surprised/     

漏洞百出的教育

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?     

Western and Eastern culture differences in commercial field

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.     

只要努力,不要后悔

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.     

浅谈移民问题

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.     

Sister Carrie, an Adherent of Desires

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.     

Improvement of one-cycle controller by use of proportional integral differential controller

1.IntroductionOne-cyclecontrolmethod,whichwasproposedaboutonedecadeago[1],hasbecomeanattractivemethodinspecialfieldssuchaspowerfactorcorrection[2-6],switchingamplifiers[7,8],etc.Themainideaofthiscontrollerisbasedonintegrationofdiodevoltageinone-cycleandforcesittobeexactlyequaltothereferencevalue.Themainadvantageofthiscontrollerisitsrealtimeabilitytorejectthevariationofinputvoltage[1].Despitethisgreatability,ithasnogoodperformancesinrejectingofloaddisturbanceandfollowingreferencecommands.Espec…     

风的曲线

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.