16, a

17, a

18, d

19, a

20, b

21, c

17 B: (động vật hoang dã)

19 A:(khí hậu mưa)

20 B(bị động và expect+to-v)

21 C(động lực)

16 A

17 A

17 D

19 A

20 B

21 C

1 am going to send

2 will get

3 will do

4 will be

5 are going to visit

6 will win

7 am going to take

8 are going

9 will go

10 is going to defeat

11 is going to have

12 will never lie

13 will fly

14 won't tell

15 will like

\(c,-\dfrac{8}{13}+\left(-\dfrac{7}{5}-x\right)=-\dfrac{1}{2}\\ -\dfrac{7}{5}-x=-\dfrac{1}{2}-\dfrac{8}{13}\\ -\dfrac{7}{5}-x=-\dfrac{29}{26}\\ x=-\dfrac{7}{5}-\left(-\dfrac{29}{26}\right)=-\dfrac{37}{130}\\ d,-1\dfrac{1}{7}-\left[-\dfrac{5}{3}+\left(x-\dfrac{7}{3}\right)\right]=-\dfrac{4}{21}\\ -\dfrac{8}{7}-\left[-\dfrac{5}{3}+\left(x-\dfrac{7}{3}\right)\right]=-\dfrac{4}{21}\\ -\dfrac{5}{3}+\left(x-\dfrac{7}{3}\right)=-\dfrac{8}{7}-\left(-\dfrac{4}{21}\right)\\ -\dfrac{5}{3}+\left(x-\dfrac{7}{3}\right)=-\dfrac{20}{21}\\ x-\dfrac{7}{3}=-\dfrac{20}{21}-\left(-\dfrac{5}{3}\right)\\ x-\dfrac{7}{3}=\dfrac{5}{7}\\ x=\dfrac{5}{7}+\dfrac{7}{3}=\dfrac{64}{21}\\ e,-\dfrac{2}{3}-x:\dfrac{1}{2}=\dfrac{2}{5}\\ x:\dfrac{1}{2}=-\dfrac{2}{3}-\dfrac{2}{5}\\ x:\dfrac{1}{2}=-\dfrac{16}{15}\\ x=-\dfrac{16}{15}\times\dfrac{1}{2}=-\dfrac{8}{15}\)

c: -8/13+(-7/5-x)=-1/2

=>x+7/5+8/13=1/2

=>x=1/2-7/5-8/13=-197/130

d: \(\Leftrightarrow-\dfrac{8}{7}+\dfrac{5}{3}-\left(x-\dfrac{7}{3}\right)=\dfrac{-4}{21}\)

=>\(x-\dfrac{7}{3}=\dfrac{-8}{7}+\dfrac{5}{3}+\dfrac{4}{21}=\dfrac{-24+35+4}{21}=\dfrac{18}{21}=\dfrac{6}{7}\)

=>x=6/7+7/3=18/21+49/21=67/21

e: =>x:1/2=-2/3-2/5=-16/15

=>x=-16/15*1/2=-8/15

f: =>-8/5*x=-1/3+4/9=1/9

=>x=-1/9:8/5=-1/9*5/8=-5/72

g: =>-4/5x-1/4+x=-13/3

=>1/5x=-13/3+1/4=-52/12+3/12=-49/12

=>x=-49/12*5=-245/12

h: =>12/7:x-1/2=0 hoặc 2/5x-3/2=0

=>12/7:x=1/2 hoặc 2/5x=3/2

=>x=12/7:1/2=24/7 hoặc x=3/2:2/5=3/2*5/2=15/4

1/

$C=5+(5^2+5^3)+(5^4+5^5)+.....+(5^{2022}+5^{2023})$

$=5+5^2(1+5)+5^4(1+5)+....+5^{2022}(1+5)$

$=5+(1+5)(5^2+5^4+....+5^{2022})$
$=5+6(5^2+5^4+....+5^{2022})$

$\Rightarrow C$ chia $6$ dư $5$

$\Rightarrow C\not\vdots 6$

2/

$D=(1+2+2^2)+(2^3+2^4+2^5)+....+(2^{2019}+2^{2020}+2^{2021})$

$=(1+2+2^2)+2^3(1+2+2^2)+....+2^{2019}(1+2+2^2)$

$=(1+2+2^2)(1+2^3+...+2^{2019})$

$=7(1+2^3+...+2^{2019})\vdots 7$ 

Ta có đpcm.

Bạn cần bài nào vậy bạn?

a: Ư(8)={1;2;4;8}

Ư(12)={1;2;3;4;6;12}

UC(8;12)={1;2;4}

b: B(16)={0;16;32;...}

B(24)={0;24;48;...}

BC(16,24)={0;48;96;...}

\(c,B\left(12\right)=\left\{0;12;24;36;48;...\right\}\\ B\left(18\right)=\left\{0;18;36;54;72;...\right\}\\ BC\left(12;18\right)=B\left(2^2.3^2\right)=B\left(36\right)=\left\{0;36;72;108;144;....\right\}\\ d,Ư\left(16\right)=\left\{1;2;4;8;16\right\}\\ Ư\left(24\right)=\left\{1;2;3;4;6;8;12;24\right\}\\ ƯC\left(16;24\right)=Ư\left(2^3\right)=Ư\left(8\right)=\left\{1;2;4;8\right\}\\ e,ƯC\left(28;70\right)=Ư\left(2.7\right)=Ư\left(14\right)=\left\{1;2;7;14\right\}\\ BC\left(4;14\right)=B\left(2^2.7\right)=B\left(28\right)=\left\{0;28;56;84;112;140;...\right\}\)