解析试题背后的真相
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> 下列四个命题：①若0＞a＞b，则1a＜1b；②x＞0，x+1x-1的最小值为3；③椭圆x24+y23=...
下列四个命题：①若0＞a＞b，则1a＜1b；②x＞0，x+1x-1的最小值为3；③椭圆x24+y23=1比椭圆x24+y22=1更接近于圆；④设A，B为平面内两个定点，若有|PA|+|PB|=2，则动点P的轨迹是椭圆；其中真命题的序号为&．（写出所有真命题的序号）
题型：填空题难度：中档来源：不详
对于①，因为b＜a＜0，两边都除以ab，得1a＜1b＜0所以①是真命题；对于②，因为x＞0，x+1x-1=（x-1）+1x-1+1≥2(x-1)?1x-1+1=3当且仅当x=2时，x+1x-1的最小值为3，所以②是真命题；对于③，因为椭圆x24+y23=1的离心率e1=12椭圆x24+y22=1的离心率e2=22，且12＜22，所以椭圆x24+y23=1比椭圆x24+y22=1更接近于圆，③是真命题；对于④，|PA|+|PB|=2，在|AB|＜2的情况下，P的轨迹才是椭圆题设中没有|AB|＜2，故④是假命题故答案为：①②③
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据好范本试题专家分析，试题“下列四个命题：①若0＞a＞b，则1a＜1b；②x＞0，x+1x-1的最小值为3；③椭..”主要考查你对&&真命题、假命题&&等考点的理解。关于这些考点的“档案”如下：
真命题、假命题
命题的概念：
1、命题：把语言、符号或式子表达的，可以判断真假的陈述句称为命题； 2、真命题、假命题：判断为真的语句称为真命题，判断为假的语句称为假命题。 注意：
1、并不是所有的语句都是命题，只有能够判断真假的语句才是命题。
2、如果一个语句是命题，则它是真命题或是假命题，二者必具其一。
发现相似题
与“下列四个命题：①若0＞a＞b，则1a＜1b；②x＞0，x+1x-1的最小值为3；③椭圆x24+y...”相似的试题有：
查阅次数试题题文
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＞＞＞计算下列各式：（1）1a-b+1a+b+2aa2+b2+4a3a4+b4；（2）x2+yzx2+(y-z)..
计算下列各式：（1）1a-b+1a+b+2aa2+b2+4a3a4+b4；（2）x2+yzx2+(y-z)x-yz+y2-zxy2+(z+x)y+zx+z2+xyz2-(x-y)z-xy；（3）x3-1x3+2x2+2x+1+x3+1x3-2x2+2x-1-2(x2+1)x2-1（4）(y-x)(z-x)(x-2y+z)(x+y-2z)+(z-y)(x-y)(x+y-2z)(y+z-2x)+(x-z)(y-z)(y+z-2x)(x-2y+z)．
题型：解答题难度：中档来源：不详
（1）1a-b+1a+b+2aa2+b2+4a3a4+b4=2aa2-b2+2aa2+b2+4a3a4+b4=4a3a4-b4+4a3a4+b4=8a7a8-b8；（2）x2+yzx2+(y-z)x-yz+y2-zxy2+(z+x)y+zx+z2+xyz2-(x-y)z-xy=x(x-z)+z(x+y)(x+y)(x-z)+y(x+y)-x(y+z)(x+y)(y+z)+z(y+z)-y(z-x)(z-x)(y+z)=xx+y+zx-z+yy+z-xx+y-zx-z-yy+z=0；（3）x3-1x3+2x2+2x+1+x3+1x3-2x2+2x-1-2(x2+1)x2-1=(x-1)(x2+x+1)(x+1)(x2+x+1)+(x+1)(x2-x+1)(x-1)(x2-x+1)-2(x2+1)(x+1)(x-1)=x-1x+1+x+1x-1-2(x2+1)(x+1)(x-1)=0；（4）设x-y=a，y-z=b，z-x=c，则(y-x)(z-x)(x-2y+z)(x+y-2z)+(z-y)(x-y)(x+y-2z)(y+z-2x)+(x-z)(y-z)(y+z-2x)(x-2y+z)=-ac(a-b)(b-c)-ab(b-c)(c-a)-cb(c-a)(c-b)=-ac(c-a)+ab(a-b)+bc(b-c)(a-b)(b-c)(c-a)=(a-b)(b-c)(c-a)(a-b)(b-c)(c-a)=1．
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据魔方格专家权威分析，试题“计算下列各式：（1）1a-b+1a+b+2aa2+b2+4a3a4+b4；（2）x2+yzx2+(y-z)..”主要考查你对&&分式的加减&&等考点的理解。关于这些考点的“档案”如下：
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因为篇幅有限，只列出部分考点，详细请访问。
分式的加减
分式的加减法则：同分母的分式相加减，分母不变，把分子相加减；异分母的分式相加减，先通分，变为同分母分式，然后再加减。 用式子表示为： 分式的加减要求：①分式的加减运算结果必须是最简分式或整式，运算中要适时地约分；②如果一个分式与一个整式相加减，那么可以把整式看成是分母为1的分式，先通分，再进行加减。
发现相似题
与“计算下列各式：（1）1a-b+1a+b+2aa2+b2+4a3a4+b4；（2）x2+yzx2+(y-z)..”考查相似的试题有：
518794206189508064210872175851882011·方程2X+A=1与方程2/1X+3=x+4的解互为倒数,求关Y的方程9-3Y=2分之1A+5的解._百度知道
1·方程2X+A=1与方程2/1X+3=x+4的解互为倒数,求关Y的方程9-3Y=2分之1A+5的解.
1·方程2X+A=1与方程2/1X+3=x+4的解互为倒数,求关Y的方程9-3Y=2分之1A+5的解。
计净羔断薏登割券公猾（字母我是打的大写）（要算式，看的懂的。谢谢，）（七年级的内容）
是2分之1X，打错了···，，，
提问者采纳
2x+A=嘁佥促缴讵剂存烯担楼1，——》x1=(1-A)/2，(1/2)x+3=x+4，——》x2=-2，x1*x2=1，——》A=2，9-3y=A/2+5=6，——》y=1。
提问者评价
谢谢了，，，，，，
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