x2+y2=4,求3xy的最小值是

2025-03-25 01:50:42

推荐回答（2个）

回答1：

已知
X²+Y²=4 ——（1）
由（X+Y）²≥0，可得
X²+Y²+2XY≥0 ——（2）
将（1）式代入（2）式，得
4+2XY≥0
得到，XY≥-2
所以 3XY≥-6
3XY的最小值是-6

回答2：

小ought catch-caught make-made read-read hurt-hurt let-let put-put set-set meet-met draw-drew
2反义词，对应词。　　
black--white girl--boy befor-- after new—old Mrs--Mr over--under big--small in--out ask—answer old--yound up--down open--close good--bad yes--no here--there right--wrong come--go this--that fat--thin light--heavy teacher--student short--long/tall clean--dirty strong-thin same--different on-under today--yestoday early--late hot--cold north--sorth warm--cool sister--brother cry--laugh busy--free far--near full--hungry light—dark sad—excited teach—study put on—take off
get off---get on go to bed---get up go there---come here
3同音词　　 high-hi there-their two-to/too four-for here-hea meet-meat whose-who’s where-wear right-write our-hour I-eye why-Yy Uu-you know-no are-Rr see-sea-Cc one-won sun-son buy-by Oo-oh aren’t-aunt
4近义词　　
photo-picture hello-hi desk-table nice-good always-often kid-child arrive-get to must-have to sure-certainly many-a lot of would like-want pretty-beautiful eigher-too gift-present
5完全—缩写形式　　
arenot=aren’t is not=isn’t do not=don’t did not=didn’t does not=doesn’t can not=can’t was not=wasn’t I will=I’ll I am=I’m I would=I’d Let us=Let‘s they are=they’re what is=what’s she is=she’s it is=it’s he is=he’s I will=I’ll we will=we’ll were not=weren’t mister=Mr centimeter=cm厘米
一，一般情况在词未直接加“s”. 如：book—books girl-girls
二，如果单词以s,x,ch,sh结尾，则在词尾加“es”.
如box-boxes dress-dresses watch-watches
三，以字母o结尾的单词，该词是有生命体加“es”，该词是无生命体加“s”。如：tomato—tomatoes(西红柿) potato—potatoes（土豆） photo—photos(照片) radio—radios(收音机)
四，以y结尾的单词，y前是元音的加s,y前是辅音的把y变成i再加es.
如：boy—boys day—days baby—babies family—families country--countries
五，以f,fe结尾的单词去掉f,fe再加ves.
如：knife-knives leaf-leaves wife—wives shelf—shelves.
六，不规则变化的名词。
foot—feet goose—geese tooth—teeth deer—deer mouse—mice fish—fish man—men Chinese—Chinese woman—women sheep—sheep child—children this—these that—those you—you Japanese—Japanese policeman—policemen is/am—are it/he/she--they
7名词→形容词。
China→Chinese Japan→Japanese America→American sun→sunny snow→snowy wind→windy fun→funny rain→rainy noise→noisy help→helpful friend→friendly excite→excited use→useful love→lovely colour→colourful
8动词→名词　　 climb→climber write→writer sing→singer drive→driver
work→worker teach→teacher clean→cleaner farm→farmer
play→player run→runner
9动词的现在分词。　　
1．一般情况加ing. 如：read→reading do→doing
2.以e结尾的单词去掉e再加ing. 如：make→making drive→driving write→writing
3.以重读闭音节结尾的单词双写末尾辅音字母再加ing.
如；swim→swimming run→running
10形容词的比较级和最高级的构成。　　单音节词的构成如下：
1．一般在词尾加er或est. 如：tall→taller long→longer
2.以e结尾的单词加er或est. 如：fine→finer late→later
3.以辅音字母加y结尾的单词把y变成i再加er或est. 如：heavy→heavier easy→easier
4.以重读闭音节结尾的单词双写末尾辅音字母再加er或est.
如：big→bigger thin→thinner fat→fatter
5.不规则变化。
good→better→best bad→worse→worst many→more→most
little→less→least far→farther→farthest
多音节词在词前加more.或most.
11动词的过去式。　　
1，一般情况加ed. 如：visit→visted learn→learned
2．以e结尾的单词直接加d.。如：make→maked write→writed
3.以辅音加y结尾的单词把y变成i再加ed。如：study→studied
以元音加y结尾的单词直接加ed. 如：play→played
3．以重读闭音节结尾的单词双写末尾辅音字母再加ed. 如：stop→stopped
12序数词的缩写形式。　　
first→1st second→2nd third→3rd fourth→4th fifth→5th eighth→8th
ninth→9th twelfth→12th twentieth→20th twenty-first→21st

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