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20 tháng 9 2021

a) Xét hai tam giác vuông ADB và DCA có

Góc ABD = DAC (cùng phụ BDC)

Nên hai tam giác trên đồng dạng (góc - góc)

b) Từ hai tam giác đồng dạng ở câu a ta có tỉ lệ:

AD/DC = AB/AD

Hay AD^2 = ABxDC = 4x9

Tính ra AD = 6cm

~ Chúc bn hok toots~ hình bn tự vẽ nhé ^^ ~!!

20 tháng 9 2021

Ùmm..c cc     xxccc 

19 tháng 9 2021

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~ Chúc bạn hok tốt ~ =)

19 tháng 9 2021

\(A=\sqrt{\left(2020-2x\right)^2}+\sqrt{\left(2019-2x\right)^2}-2\)

\(=\left|2020-2x\right|+\left|2019-2x\right|-2\)

\(=\left|2020-2x\right|+\left|2x-2019\right|-2\)

\(\ge\left|2020-2x+2x-2019\right|-2=\left|1\right|-2=-1\)

Dấu "=" xảy ra <=> ( 2020 - 2x )( 2x - 2019 ) ≥ 0 <=> 2019/2 ≤ x ≤ 1010

Vậy MinA = -1

19 tháng 9 2021

a, sinC = \(\frac{AB}{BC}\); tanC = \(\frac{AB}{AC}\)

cosC = \(\frac{AC}{BC}\); cotC = \(\frac{AC}{AB}\)

b, Xét tam giác ABC vuông tại A, đường cao AH

tanB = \(\frac{AC}{AB}=\sqrt{2}\Rightarrow AC=\sqrt{2}AB\)

* Áp dụng hệ thức : \(\frac{1}{AH^2}=\frac{1}{AB^2}+\frac{1}{AC^2}\Rightarrow\frac{1}{12}=\frac{1}{AB^2}+\frac{1}{2AB^2}\Rightarrow AB\approx4,24\)cm 

\(\Rightarrow AC\approx4,24\sqrt{2}\)cm

Theo định lí Pytago tam giác ABC vuông tại A

\(BC=\sqrt{AB^2+AC^2}\approx\sqrt{4,24^2+\left(4,24\sqrt{2}\right)^2}\approx7,34\)cm 

18 tháng 9 2021

Ta có: 

\(y=3x\left(k^2-k+1\right)=-2\)

\(\Rightarrow3\left(-3\right)\left(k^2-k+1\right)=-2\)

\(\Rightarrow-9\left(k^2-k+1\right)=-2\)

\(\Rightarrow k^2-k+1=\frac{2}{9}\)

\(\Rightarrow\left(k-\frac{1}{2}\right)^2=\frac{2}{9}-\frac{3}{4}=\frac{35}{36}\) (Vô nghiệm)

1) I haven’t seen Mike………he gave me his calculator.A. since B. for C. until D. before2) …….it is raining, he still waits for her.A. If B. Unless C. Although D. After3) ………John has a headache, he has to take an aspirin.A. So B. As long as C. In order to D. Because4) She had left…….we came.A. when B. by the time C. while D. since5) I moved to the first row……I could see better.A. such B. so C. so that D. such that6) He works hard………help his wife.A. in order that B....
Đọc tiếp

1) I haven’t seen Mike………he gave me his calculator.

A. since B. for C. until D. before

2) …….it is raining, he still waits for her.

A. If B. Unless C. Although D. After

3) ………John has a headache, he has to take an aspirin.

A. So B. As long as C. In order to D. Because

4) She had left…….we came.

A. when B. by the time C. while D. since

5) I moved to the first row……I could see better.

A. such B. so C. so that D. such that

6) He works hard………help his wife.

A. in order that B. in order to C. so as that D. so that

7) Look! She is wearing beautiful dress.

Yup. She always wears……clothes.

A. such lovely B. so lovely C. such a lovely D. as lovely as

8) ……..rain or snow, there are always many people at this concert.

A. although B. since C. despite D. despite of

9) I never bring along rain coat…….it’s raining heavily.

A. unless B. if C. therfore D. so as to

10) I got a new watch……I could do the exam on time.

A. although B. till C. since D. so that

11.If you want to join this club, you must_______ this application form.

a. make up b. write down c. do up d. fill in

12. The water company will have to _______ off water supplies while repairs to the pipes

are carried out.

a. take b. cut c. set d. break

13. Come_______, children! Get your coats on or you'll be late for school.

a. along b. to c. across d. over

14. She tried to prevent the dog_______ running into the road.

a. to b. for c. from d. against

15. The doctor told him to keep_______ sweets and chocolate to lose weight.

a. at b. up c. back d. off

16. Tom was slow to catch_______, but gradually he began to understand.

a. in b. on c. away d. out

17. I haven’t time to speak to him now, you'll have to put him_______

a. off c. back c. aside d. away

18. You know I'll always stand_______ you if you are in trouble.

a. for b. up c. by d. with

19. I disapprove_______ people smoking in public places.

a. on b. at c. with d. of

20. She found it is hard to _______ up to the fact that she would never be famous.

a. come b. face c. get d. keep

21. For the first few minutes she was leading the race, then she began to fall_______

a. out b. through c. back d. off

22. The job offer was too good for him to turn_______

a. down b. out c. off d. away

23. His son's death was a terrible shock and it took him a long time to _______ it.

a. get round b. come through c. go over d. get over

24. Please come_______ and see us some time-you're always welcome.

a. to b. round c. about d. away

25. She put _______ speaking to him as long as possible.

a. over b. away c. off d. back

26. He filled in the necessary forms and _______ for the job.

a. appealed b. asked c. requested d. applied

27. She went _______ a bad cold just before Christmas.

a. down with b. in for c. over d. through

28. They had never come_______ such a beautiful little village before.

a. at b. across c. off d. long

29. When the tenants failed to pay their bills, the authorities decided to cut_______ the gas

supply to the flats.

a. out b. down c. off d. across

30. The doctor advised me not to take_______ so much work in future.

a. to b. over c. after d. On

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18 tháng 9 2021

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18 tháng 9 2021

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