1: Áp dụng tính chất của DTSBN, ta được:

\(\dfrac{x}{7}=\dfrac{y}{13}=\dfrac{x-y}{7-13}=\dfrac{42}{-6}=-7\)

=>x=-48; y=-91

2: x/y=3/4

=>4x=3y

=>4x-3y=0

mà 2x+y=10

nên x=3 và y=4

3: =>7x-3y=0 và x-y=-24

=>x=18 và y=42

4: =>7x-5y=0 và x+y=24

=>x=10 và y=14

Bài 3. 

\(\left\{{}\begin{matrix}a\left(a+b+c\right)=-\dfrac{1}{24}\left(1\right)\\c\left(a+b+c\right)=-\dfrac{1}{72}\left(2\right)\\b\left(a+b+c\right)=\dfrac{1}{16}\left(3\right)\end{matrix}\right.\)

Dễ thấy \(a,b,c\ne0\Rightarrow a+b+c\ne0\)

Chia (1) cho (2), ta được \(\dfrac{a}{c}=3\Rightarrow a=3c\left(4\right)\)

Chia (2) cho (3) ta được: \(\dfrac{c}{b}=-\dfrac{2}{9}\Rightarrow b=-\dfrac{9}{2}c\left(5\right)\).

Thay (4), (5) vào (2), ta được: \(-\dfrac{1}{2}c^2=-\dfrac{1}{72}\)

\(\Rightarrow c=\pm\dfrac{1}{6}\).

Với \(c=\dfrac{1}{6}\Rightarrow\left\{{}\begin{matrix}a=3c=\dfrac{1}{2}\\b=-\dfrac{9}{2}c=-\dfrac{3}{4}\end{matrix}\right.\)

Với \(c=-\dfrac{1}{6}\Rightarrow\left\{{}\begin{matrix}a=3c=-\dfrac{1}{2}\\b=-\dfrac{9}{2}c=\dfrac{3}{4}\end{matrix}\right.\)

Vậy: \(\left(a;b;c\right)=\left\{\left(\dfrac{1}{2};-\dfrac{3}{4};\dfrac{1}{6}\right);\left(-\dfrac{1}{2};\dfrac{3}{4};-\dfrac{1}{6}\right)\right\}\)

1, a+b-c = 18 với b=10 ; c=-9

Với b=10 ; c=-9 ta có:

a+b-c = a+10-(-9) 

=> a+10-(-9) = 18

<=> a+10+9=18

<=> a=18-10-9

<=> a=-1

Vậy a = -1

2, 2a - 3b + c = 0 với b = -2 ; c = 4

Với b=-2 ; c=4 ta có:

2a - 3b + c = 2a - [3.(-2)] + 4

=> 2a - [3.(-2)] + 4 = 0

<=> 2a - (-6) + 4 = 0

<=> 2a + 6 + 4 = 0

<=> 2a = 0-6-4

<=> 2a = -10

<=> a = (-10) : 2

<=> a = -5

Vậy a = -5

k mình nha

1) Thay b= 10; c = -9 vào biểu thức, ta có:

\(a+10-\left(-9\right)=18\)

\(a=18-10-9\)

\(a=-1\)

2) Thay b = -2; c= 4 vào biểu thức ta có:

\(2a-3.\left(-2\right)+4=0\)

\(2a+10=0\)

\(2a=-10\)

\(a=-5\)

3) Thay b = 6; c= -1 vào biểu thức ta có:

\(3a-6-2.\left(-1\right)=2\)

\(3a-4=2\)

\(3a=6\)

\(a=2\)

b) Thay b = -7; c= 5 vào biểu thức ta có:

\(12-a+\left(-7\right)+5.5=-1\)

\(12-a+18=-1\)

\(12-a=-19\)

\(a=-7\)

5) Thay b = -3; c= -7 vào biểu thức ta có:

\(1-2.\left(-3\right)+\left(-7\right)-3a=-9\)

\(-3a=-9\)

\(a=3\)

hok tốt!!

xin lỗi câu 2 mik sai phải là -5 mới đúng á

1.1. Mức độ nhận biết

1. Maryam wanted___________ the mosque on Hang Luoc Street.

A. to visit                    B. visit                         C. visited                     D. visiting

2. Do you enjoy___________other countries?

A. to visit                    B. visit                         C. visited                     D. visiting

3. People must___________something to protect our environment.

A. doing                      B. did                C. do                         D. to do

4. The examiner asked him___________many questions about his hobby.

A- to answer               B- answering               C- answered                D- answers

5. Let’s ___________ Hoa in the hospital to cheer her up.

A. to visit                    B. visitedC. visitingD. visit

6. What about ___________ to HCM city for our holiday?

A. to goB. goingC. goD. goes

7.We decided_________a house with a swimming pool.

  A. buys                                  B. to buy                     C. bought                    D. to buying

8. Why don’t we ___________ out and have some fun this weekend?

A. goingB. goC. to goD. goes

9. They are not old enough___________drive a car to school.

 A. drive                                  B. to drive                   C. drove                      D. driving

10. My dad doesn’t mind ___________ my mom from work every day.

 A. pick up                  B. picked up                C. picking up   D. to pick up

11. What should you___________if you face this problem?

 A. do                                      B. doing                      C. to do                       D. does

12. Wearing uniforms encourages students___________proud of their school because the uniforms bear their schools’ name.

 A. to be                                  B. be                            C. being                       D. been

13. Tom avoids ___________his lessons as long as possible.

A. study B. studying               C. to study                 D. studied

14. I finally finished___________at 7p.m, and served dinner.

 A. cooking                             B. being cooked           C. to cook                  D. to be cooked

15. Let me ___________ from you soon.

A. to hear                              B. hear                       C. hearing                        D. heard

16. Ha loves ___________ with her mum on Sunday

A. go shop                   B.to go shop               C. going shoppingD.going shop

17. My students enjoy ___________ English very much.

 A. learn                      B. learning       C. to learn                   D. learnt

18. Please let Tom ___________with you.

A. go                                   B. going                  C. to go                    D. goes

19. Haven't you finished___________ the answer?

A. write                                 B. writing                  C. how to write           D.  them to write

23. How about___________out for luch?

 A. go                            B. going                          C. to go                    D. goes

24. It’s noteasy___________pass the final exam without working hard.

 A. passing                  B. pass                        C. to pass                    D. passed

25. The teacher told her students ___________talking.

   A- stop                     B- stopped                  C- to stop                                D- stopping

26. You cant stop me from _____ what I want.        

 A. doing                     B. do                     C. to do                 D. that I do

28. He tried to  ______ my question.                   

A. answering               B. to answer                C. answer                    D. how to answer

29. There’s no point in  him to do that.

A. persuade     B. persuadingC. persuaded     D.  to persuade

30. Tom offered…………..Jane a ticket to the theater, but she refused to take it.

1.1. Mức độ nhận biết

1. Maryam wanted___________ the mosque on Hang Luoc Street.

A. to visit                    B. visit                         C. visited                     D. visiting

2. Do you enjoy___________other countries?

A. to visit                    B. visit                         C. visited                     D. visiting

3. People must___________something to protect our environment.

A. doing                      B. did                C. do                         D. to do

4. The examiner asked him___________many questions about his hobby.

A- to answer               B- answering               C- answered                D- answers

5. Let’s ___________ Hoa in the hospital to cheer her up.

A. to visit                    B. visitedC. visitingD. visit

6. What about ___________ to HCM city for our holiday?

A. to goB. goingC. goD. goes

7.We decided_________a house with a swimming pool.

  A. buys                                  B. to buy                     C. bought                    D. to buying

8. Why don’t we ___________ out and have some fun this weekend?

A. goingB. goC. to goD. goes

9. They are not old enough___________drive a car to school.

 A. drive                                  B. to drive                   C. drove                      D. driving

10. My dad doesn’t mind ___________ my mom from work every day.

 A. pick up                  B. picked up                C. picking up   D. to pick up

11. What should you___________if you face this problem?

 A. do                                      B. doing                      C. to do                       D. does

12. Wearing uniforms encourages students___________proud of their school because the uniforms bear their schools’ name.

 A. to be                                  B. be                            C. being                       D. been

13. Tom avoids ___________his lessons as long as possible.

A. study B. studying               C. to study                 D. studied

14. I finally finished___________at 7p.m, and served dinner.

 A. cooking                             B. being cooked           C. to cook                  D. to be cooked

15. Let me ___________ from you soon.

A. to hear                              B. hear                       C. hearing                        D. heard

16. Ha loves ___________ with her mum on Sunday

A. go shop                   B.to go shop               C. going shoppingD.going shop

17. My students enjoy ___________ English very much.

 A. learn                      B. learning       C. to learn                   D. learnt

18. Please let Tom ___________with you.

A. go                                   B. going                  C. to go                    D. goes

19. Haven't you finished___________ the answer?

A. write                                 B. writing                  C. how to write           D.  them to write

23. How about___________out for luch?

 A. go                            B. going                          C. to go                    D. goes

24. It’s noteasy___________pass the final exam without working hard.

 A. passing                  B. pass                        C. to pass                    D. passed

25. The teacher told her students ___________talking.

   A- stop                     B- stopped                  C- to stop                                D- stopping

26. You cant stop me from _____ what I want.        

 A. doing                     B. do                     C. to do                 D. that I do

28. He tried to  ______ my question.                   

A. answering               B. to answer                C. answer                    D. how to answer

29. There’s no point in  him to do that.

A. persuade     B. persuadingC. persuaded     D.  to persuade

30. Tom offered…………..Jane a ticket to the theater, but she refused to take it.

thay số vào ta có

thôi bạn

nên cái này bạn

nên tự làm 

1) a + b - c = 18 

    a + 10 + 9 = 18

     a  + 10   = 18 - 9

     a  + 10  = 9

             a = 9 - 10

             a = -1

2) 2a - 3b + c = 0

    2a - 3 .(-2) + 4 =0

     2a + 6  +4  = 0

     2a  + 6  = 0 - 4

      2a + 6 = -4

             2a = -4 -6

             2a = -10

              a = -5

3) 3a - b - 2c = 2

     3a - 6 - 2 .(-1) = 2

      3a - 6 + 2  =2

       3a - 6   =  2-2

       3a  = 6

         a  = 2