1. I have written the letter for 2 days
2.My father hasn't drunk wine for a year
3.She hasn't read a newspaper since May12
4.Jack has been a reporter since 2010
5.Mary began learning English 6 years ago
6. It’s 10 years since we went to the city
7.His father began teaching 5 years ago
8.She stopped  writing to John since 2008
9.The last time my ucle taught in this school was five years ago
10.It’s ages since I worked for an international bank

11.I haven't corressponded with John a long time ago
12.Linda hasn't been talked to me for many years
13.You started taking part in this 
14.We last saw Maty since we left the university
15.You last helped me when you graduated from the college
16.He hasn't gone abroad before
17.She has driven for 1 month
 

De mih !!

may có tao nha

Bai 2 :
1.speaking
2.famous
3.entertainment
4. second
5.sunny
6.better
7.longest
8.nationality
9.electricity
10.himself

\(\dfrac{3a-2b}{5}=\dfrac{2c-5a}{3}=\dfrac{5b-3c}{2}=\dfrac{6c-15a}{9}=\dfrac{10b-6c}{4}=\dfrac{6c-15a+10b-6c}{9+4}=\dfrac{10b-15a}{13}\)

\(\Rightarrow\dfrac{3a-2b}{5}=\dfrac{10b-15a}{13}\Rightarrow39a-26b=50b-75a\)

\(\Rightarrow114a=76b\Rightarrow b=\dfrac{3}{2}a\)

Tương tự:

\(\dfrac{2c-5a}{3}=\dfrac{3a-2b}{5}=\dfrac{5b-3c}{2}=\dfrac{15a-10b}{25}=\dfrac{10b-6c}{4}=\dfrac{15a-10b+10b-6c}{25+4}=\dfrac{15a-6c}{29}\)

\(\Rightarrow\dfrac{2c-5a}{3}=\dfrac{15a-6c}{29}\Rightarrow58c-145a=45a-18c\)

\(\Rightarrow76c=190a\Rightarrow c=\dfrac{5}{2}a\)

Thế vào \(a^2+275=bc\)

\(\Rightarrow a^2+275=\left(\dfrac{3}{2}a\right).\left(\dfrac{5}{2}a\right)=\dfrac{15}{4}a^2\)

\(\Rightarrow\dfrac{11}{4}a^2=275\Rightarrow a^2=100\Rightarrow a=\pm10\)

- Với \(a=10\Rightarrow b=\dfrac{3}{2}a=15\) ; \(c=\dfrac{5}{2}a=25\)

- Với \(a=-10\Rightarrow b=-15;c=-25\)

GIÚP MIK VS NHA:(((((

CẢM ƠN RẤT NHIỀU

MN XONG CÂU NÀO THÌ CỨ GỬI LUÔN CHO MIK CÂU ĐÓ NHA;-;

MIK CÒN CHÉP KỊP

:(((((((((((((( NHANHH NHANH GIÚP MIK Ạ

Câu 1:

\(a,\dfrac{x}{4}=\dfrac{y}{7}=\dfrac{x-y}{4-7}=\dfrac{-15}{-3}=5\\ \Rightarrow\left\{{}\begin{matrix}x=20\\y=35\end{matrix}\right.\\ b,\dfrac{x}{3}=\dfrac{y}{5}=\dfrac{x+y}{3+5}=\dfrac{-32}{8}=-4\\ \Rightarrow\left\{{}\begin{matrix}x=-12\\y=-20\end{matrix}\right.\\ c,\dfrac{x}{2}=\dfrac{y}{3}=\dfrac{z}{5}=\dfrac{x+y+z}{2+3+5}=\dfrac{-90}{10}=-9\\ \Rightarrow\left\{{}\begin{matrix}x=-18\\y=-27\\z=-45\end{matrix}\right.\\ d,\dfrac{x}{4}=\dfrac{y}{2}=\dfrac{z}{7}=\dfrac{2x-4y+3z}{8-8+21}=\dfrac{42}{21}=2\\ \Rightarrow\left\{{}\begin{matrix}x=8\\y=4\\z=14\end{matrix}\right.\)

\(e,\dfrac{x}{5}=\dfrac{y}{6}=\dfrac{z}{7}=\dfrac{z-x}{7-5}=\dfrac{30}{2}=15\\ \Rightarrow\left\{{}\begin{matrix}x=75\\y=90\\z=105\end{matrix}\right.\\ f,\Rightarrow\dfrac{x}{3}=\dfrac{y}{5};\dfrac{x}{4}=\dfrac{z}{3}\Rightarrow\dfrac{x}{12}=\dfrac{y}{20}=\dfrac{z}{9}=\dfrac{x-y-z}{12-20-9}=\dfrac{-68}{-17}=4\\ \Rightarrow\left\{{}\begin{matrix}x=48\\y=80\\z=36\end{matrix}\right.\\ g,\Rightarrow\dfrac{x}{6}=\dfrac{y}{4}=\dfrac{z}{3}=\dfrac{x+y+z}{6+4+3}=\dfrac{65}{13}=5\\ \Rightarrow\left\{{}\begin{matrix}x=30\\y=20\\z=15\end{matrix}\right.\\ h,\Rightarrow\dfrac{x}{4}=\dfrac{y}{6};\dfrac{y}{5}=\dfrac{z}{8}\Rightarrow\dfrac{x}{20}=\dfrac{y}{30}=\dfrac{z}{48}=\dfrac{5x-3y-3z}{100-90-144}=\dfrac{-536}{-134}=4\\ \Rightarrow\left\{{}\begin{matrix}x=80\\y=120\\z=192\end{matrix}\right.\)

a) \(=x\left(x-y\right)+\left(x-y\right)=\left(x-y\right)\left(x+1\right)\)

b) \(=a^2\left(a-x\right)-y\left(a-x\right)=\left(a-x\right)\left(a^2-y\right)\)

c) \(=3\left(x^2+4x+4\right)=3\left(x+2\right)^2\)

d) \(=2\left(a^2-b^2\right)-5\left(a-b\right)=2\left(a-b\right)\left(a+b\right)-5\left(a-b\right)\)

\(=\left(a-b\right)\left(2a+2b+5\right)\)

e) \(=xy\left(x-y\right)-3\left(x^2-y^2\right)=xy\left(x-y\right)-3\left(x-y\right)\left(x+y\right)\)

\(=\left(x-y\right)\left(xy-3x-3y\right)\)

f) \(=x^2\left(x+5\right)-4\left(x+5\right)=\left(x+5\right)\left(x^2-4\right)\)

\(=\left(x+5\right)\left(x-2\right)\left(x+2\right)\)

\(0,5x-\frac{2}{3}x=\frac{5}{12}\)

\(\frac{1}{2}x-\frac{2}{3}x=\frac{5}{12}\)

\(x.\left(\frac{1}{2}-\frac{2}{3}\right)=\frac{5}{12}\)

\(\Rightarrow x=-\frac{5}{2}\)

đúng chứ bn