x-y=9\(\Rightarrow x=9+y\) thế x=9+y  vào 49x-5y=485 ta được

\(49.\left(9+y\right)-5y=485\Leftrightarrow441+49y-5y=485\Leftrightarrow49y-5y=44\Leftrightarrow44y=44\Rightarrow y=1\)

thế y=1 vào x-y=9 ta được

x-1=9

=>x=10 

vậy...

C:2 

Thay \(x-y=9\)vào \(49x-5y=485\)có :

\(44x+\left(5x-5y\right)=485\)

\( < =>44x=485-45=440\)

\(< =>x=10\)

Thay \(x=10\)vào \(x-y=9\)có :

\(10-y=9\)

\(< =>y=1\)

Vậy ...

\(\hept{\begin{cases}4x-y=5\left(1\right)\\16y^2-8xy+x^2-40xy+10x+25=0\left(2\right)\end{cases}}\)

(1) thay (2) => \(\left(4x-5\right)^2-8x\left(4x-5\right)^2+x^2-40x\left(4x-5\right)+10x+25=0\)

\(\Leftrightarrow16x^2-40x+25-32x^2+40x+x^2-160x^2+200x+10x+25=0\)

\(\Leftrightarrow-175x^2+210x+50=0\)

\(\Leftrightarrow\orbr{\begin{cases}x=\frac{21+\sqrt{791}}{35}\Rightarrow y=\frac{-91+4\sqrt{791}}{35}\\x=\frac{21-\sqrt{791}}{35}\Rightarrow y=-\frac{91+4\sqrt{791}}{35}\end{cases}}\)

a, do x+y=30 và xy=221 nên u và v là nghiệm của pt :

 x²-30x+221=0

\(\Delta^,\)=225-221=4                     ;\(\sqrt{\Delta^,}\)=2

=> pt có hai nghiệm phân biệt .

x₁=13                   ; x₂=17

Vậy x=13;y=17 hoặc x=17; y=13

la ricardomilos

\(\hept{\begin{cases}39x+2y=400\\x+y+x^2=115\end{cases}}\)

\(\hept{\begin{cases}39x=400-2y\\x+y+x^2=115\end{cases}}\)

\(\hept{\begin{cases}x=400-\frac{2y}{39}\\x+y+x^2=115\end{cases}}\)

Ta thay 400 - 2y/39 vào biểu thức x + y + x^2 ta đc

\(400-\frac{2y}{39}+y+\left(400-\frac{2y}{39}\right)^2=115\)

\(400-\frac{41y}{39}+4\left(200-\frac{y}{39}\right)^2=115\)

\(400-4y+4\left(4000-\frac{400y}{39}+\frac{y^2}{1521}\right)=115\)

\(400-4y+16000-\frac{1600y}{39}+\frac{4y^2}{1521}=115\)

\(16400-4y-\frac{1600y}{39}+\frac{4y^2}{1521}=115\)

\(4y-\frac{1600y}{39}+\frac{4y^2}{1521}=16285\)

\(4y-\frac{1600y}{39}+\frac{4y^2}{1521}-16285=0\)

\(56316y-4y^2+24769485=0\)

Vậy hpt vô nghiệm :) 

\(\hept{\begin{cases}39x+2y=40\\x+y+x^2=115\end{cases}}\)

\(< =>\hept{\begin{cases}400-2y=39x\\x+y+x^2=115\end{cases}}< =>\hept{\begin{cases}x=\frac{400-2y}{39}\\\frac{400-2y}{39}+y+\left(\frac{400-2y}{39}\right)^2=115\end{cases}}\)

\(< =>\frac{400-2y}{39}+\frac{39y}{39}+\frac{400^2-2.400.2y+4y^2}{39^2}=\frac{4485}{39}\)

\(< =>400-2y+39y+\frac{160000-1600y+4y^2}{39}=4485\)

\(< =>37y+\frac{4\left(40000-400y+y^2\right)}{39}=4085\)

\(< =>\frac{1443y+4\left[40000-y\left(400-y\right)\right]}{39}=\frac{159315}{39}\)

\(< =>1443y+160000+1600y+4y^2=159315\)

\(< =>3043y+4y^2=-685\)\(< =>4y^2+3043y-685=0\)

Ta có : \(\Delta=3043^2-4.4.\left(-685\right)=9259849+10960=9270809\)

Do delta > 0 nên pt có 2 nghiệm phân biệt !

\(y_1=\frac{-3043+\sqrt{9270809}}{8}=0,2250402=0,225\)

\(y_2=\frac{-3043-\sqrt{9270809}}{8}=-760,9750402=-760,975\)

Vậy ...

Bạn viruss corona phân tích sai thì có xD 

Toàn lấy từ trên trời xuống cái j ko đâu hết !

c1:\(\hept{\begin{cases}5x+8y=65\\x-y=0\end{cases}}\)

\(< =>\hept{\begin{cases}5x-5y+13y=65\\5x-5y=0\end{cases}}\)

\(< =>13y=65< =>y=\frac{65}{13}=5\)

Từ \(x-y=0< =>x=y< =>x=y=5\)

c2: \(\hept{\begin{cases}5x+8y=65\\x-y=0\end{cases}}\)

\(< =>\hept{\begin{cases}5x+8y=65\\x=y\end{cases}< =>5x+8x=65< =>x=\frac{65}{13}=5}\)

\(< =>x=y=5\)

I. Choose the word / phrase (A, B, C or D) that best fits the space in each sentence.
1. Tornadoes can suck up anything ________ is in their path.
A. what B. who C. that D. where
2. When the shaking first started, Yuki thought that it ______ just another standard aftershock.
A. is B. will be C. has D. was
3. I couldn’t sleep _______________________ very tired.
A. despite I was B. in spite of I was C. because of D. although I was
4. The weather _______________ for tomorrow is very worrying.
A. news B. foresee C. forecast D. foreman
5. He is always polite and _________________ towards his employees.
A. priority B. composes C. nominates D. considerate
6. Those buckets must be filled _______________ water for future use.
A. into B. on C. with D. by
7. Mary:” I’d to congratulate you on your drawing, Tom! “  Tom: ________________
A. Yes, I think so C. It’s very good. B. Thanks, it’s O.K D. It’s very nice of you to say so
8. Tet is a festival which occurs ________ late January or early February.
A. at B. from C. on D. in
9. Scientists can usually ________ when a volcano will erupt.
A. mean B. estimate C. report D. predict
10. He is famous for his _________ . He is very helpful to those less fortunate than himself.
A. kind B. unkind C. kindly D. kindness
11. After a while Yuki realized that something __________ wrong once the shaking did not stop.
A. is B. has been C. was D. will be
12. Tet is a festival which _______________________in late January or early February.
A. holds B. celebrates C. decorates D. occurs
13. In Australia, a 120-km tropical storm is known as a ______________________.
A. hurricane B. tornado C. cyclone D. typhoon
14. Passover is celebrated by ____________________people.
A. English B. Vietnamese C. Japanese D. Jewish
15. Auld Lang Syne is a song ____________________is sung on New Year’s Eve.
A. when B. which C. where D. it
16. ______________Mr Pike is old and weak, he goes jogging every day.
A. Therefore B. Because C. Although D. However
17. Most earthquakes occur ____________________the Pacific Rim.
A. along B. around C. through D. over
18. His sense of humor distinguishes him _____________________others.
A. among B. between C. from D. with
19. “I suggest going to the seaside on the weekend.” – “_____________________.”
A. That’s a reason B. That’s a trip C. That’s a fine day D. That’s a good idea
20. Many people were injured when the building ______________ .
A. collapsed B. destroyed C. struck D. erupt
21. We will arrive ________ the morning of the sixth.
A. in B. for C. on D. at
22. Tuan was an effective ____ in that volunteer program. He spent his free time heping people.
A. active B. activist C. actor D. activity
23. The ___________ of the volcano was a terrible disaster .
A. prediction B. eruption C. cyclone D. shift
24. Thank you for your donation. It was very generous________________ you.
A. in B. at C. since D. of
25. Are they preparing for a military _________ to celebrate National Day?
A. parade B. celebration C. exhibition D. nomination
26. When tropical storm reaches 120 kilometers per hour, it is called a ___________ in America.
A. cyclone B. hurricane C. tornado D. typhoon
27. They went on walking _____________ it started to rain heavily.
A. so B. though C. because D. despite
28. The women and animals _____________ you saw on TV were from China.
A. that B. which C. whom D. who
29. “Thank you very much for that!” - “_______________.”
A. of course B. thank you C. I don’t mind D. Not at all
30. Let me congratulate you ______________ your nomination.
A. about B. for C. with D. on

31. ___________ Easter Day , many children and adults enjoy painting eggs.
A. On B. At C. During D. In
32. My friend Nigel, _____________ works in the City Bank, earns much more than I do.
A. that B. which C. who D. whose
33. “I suggest going to London for the weekend.”
A. That‘s fine day B. That‘s a good idea. C. That’s a trip. D. That‘s reason.
34. ________________ the man felt very tired, he did not stop working.
A. Though B. Because C. If D. Despite
35. ___________ a sudden shaking of the earth's surface that often causes a lot of damage.
A. Earthquake B. Tidal wave C. Typhoon D. Tornado
36. I think that nothing is more precious than __________________.
A. acquaintance B. parade C. damage D. freedom
37. Easter is a joyful festival which ____________ in late March or early April.
A. celebrates B. holds C. organizes D. occurs
38. Hurricane Andrew, _____swept through southern Florida in 1992, killed 41 people.
A. which B. that C. who D. both A and B
39. “That’s a delicious cake, my daughter.” “Thanks, Mom._____________.”
A. I’m a clever girl. B. Well done. C. It is very kind of you to say so. D. Yes, of course