đặt 

 a/b=c/d =k

=> a=b.k, c=d.k

thay vào 2 vế ta được đpcm

Có $a^2+b^2+c^2+d^2+e^2=(a+b)^2+(c+d)^2+e^2-2ab-2cd$

$=(a+b+c+d)^2+e^2 -2.(a+b)(c+d)-2ab-2cd$

$=(a+b+c+d+e)^2-2.(a+b+c+d).e-2.(a+b)(c+d)-2ab-2cd$

Mà $a^2+b^2+c^2+d^2+e^2\vdots 2;-2.(a+b+c+d).e-2.(a+b)(c+d)-2ab-2cd \vdots 2$ nên $(a+b+c+d+e)^2 \vdots 2$

Suy ra $a+b+c+d+e \vdots 2$

$a;b;c;d;e$ nguyên dương nên $a+b+c+d>2$

suy ra $a+b+c+d+e$ là hợp số

Đề yêu cầu gì vậy cậu?

chọn từ âm khác bt

nhanh nha

1A

2D

3A

4C

5A

6C

7C

8D

Sai ngữ pháp tiếng anh rồi. Khi mệnh đề If nằm sau main clause thì không có dấu phẩy .......

Có: \(\frac{a}{b}< \frac{c}{d}\Leftrightarrow ad=bc\)

\(\Leftrightarrow ad+ab< bc+ab\)

\(\Leftrightarrow a\left(b+d\right)< b\left(a+c\right)\)

\(\Leftrightarrow\frac{a}{b}< \frac{a+c}{b+d}\) (1)

Tương tự: \(ad< bc\)

\(\Leftrightarrow ad+cd< bc+cd\)

\(\Leftrightarrow d\left(a+c\right)< c\left(b+d\right)\)

\(\Leftrightarrow\frac{a+c}{b+d}< \frac{c}{d}\) (2)

Từ (1) và (2) suy ra: \(\frac{a}{b}< \frac{a+c}{b+d}< \frac{c}{d}\left(dpcm\right)\)

Chọn từ đọc khác :

A. coral B. ancient C. veteran D. allow

B khác

B khác vì phát âm là ˈānCHənt

Old man:  Can you please do me a favor?   Ba: ...............

       A. Yes. Please help me                                       B. Sure. How can I help you?

       C. Yes, you can                                                     D. No, you can't

 Câu 7. The children can look after ............................... for a few days.

       A. them                           B. they                          C. themself                   D. themselves

 Câu 8. My grandfather used ____________ us to the zoo when he was alive.

       A. took                             B. to take                     C. taking                       D. take

 Câu 9. Choose the underlined word or phrase in each sentence  that  needs correcting.

         The armchair is in the living-room among the television and the sofa.

                       A        B                                        C                                 D

Câu 10. My hometown is getting more modern and .....................

       A. more noisy                B. noisier                     C. noisiest                     D. more noisier

 Câu 11. The water wasn't ____________ in.

       A. clear enough for swimming                         B. enough clear for swimming

       C. enough clear to swim                                    D. clear enough to swim

 Câu 12. Choose the word that has the underlined part pronounces  differently

       A. looked                        B. phoned                   C. missed                      D. watched

 Câu 13. Don't come in. Pleased wait ____________ for your turn.  

       A. outside                       B. inside                       C. downstairs              D. upstairs

 Câu 14. We must be there....................7.30 and 10.30.

       A. between                     B. at                              C. after                          D. before

 Câu 15. His father is very proud  ________  him.

       A. on                                B. about                       C. of                               D. with

 Câu 16. They …………….. in that house for 3 years.

       A. have lived                  B. live                            C. lived                          D. are living   

 Câu 17. Choose the word that has a different stress pattern from the others.

       A. plentiful                      B. generous                C. volunteer                 D. sociable