Question

已知三角形ABC与三角形ADE都是等边三角形，CD=BF，求证：四边形CDEF是平行四边形

Answer 1

图中除ABCD外，还有BEDF AFCE GEFH
因为EF只是过中点O的直线，所以并不平行AB与CD
先说BEDF，因为EF过O点，所以DE与BF相等，且ED与BF平行，因此BEDF为平行四边形，同理AFCE
GEFH，因为AFCE为平行四边形，所以AF平行于CE，即GF与EH平行，同理GE与FH平行，因此GEFH为平行四边形

Answer 2

连接BE, 因为△ABC与△ADE是等边三角形, 所以 AB=AC AD=AE
角EAB=60-角BAD=角CAD
△ABE≌△ACD
角ACD=角ABE=60度 CD=BE
因为CD=BF 所以△BEF是全等三角形, 则CD=EF
因为 角BAC=ABC, AC=AB BD=BC-CD=AB-BF=AF
所以 △ACF≌△ABD
AD=CF 又AD=DE, 所以CF=DE
因此 四边形CDEF是平行四边形

Answer 3

已知△ABC与△ADE都是等边三角形，CD＝BF，求证： 四边形CDEF是平行四边形图中除ABCD外，还有BEDF AFCE GEFH 因为EF只是过中点O的直线，所以并不

Answer 4

当谓语是过去时时，助动词要用did。助动词do的第三人称单数现在时为does，过去时为did，它的用法如下：一、构成一般现在时、一般过去时的否定和疑问形式。Does he come from England?No, he doesn't come from England.Did you bring the textbook?No, I didn't bring it.二、当否定字、only加状语或so等程度副词位于句首时，要把助动词do、does、did放在主语的前面，后面的谓语动词用原形动词。如：Never does he go to school late.Rarely does he tell a joke.Not until I talked to him, did I know he was homesick.No hope did I have at that time.Not a word did my father say.Not only does he draw illustrations, but he also writes stories.Only when it rains, do you feel cool.Well do I remember the promise you made last year.His sister spent the whole evening watching TV, and so did he.三、替代重复的动词。I often donate money to charity, and so does my wife.You don't feel like eating any more, and neither do I.Do you smoke? Yes, I usually do.I know I should go on a diet, but I never do.四、加在动词原形前，表示强调。I always do arrive on time.I did tell you.The letter we had expected never did arrive.My parents don't think I studied for my exams, but I did study.