Ta có:\(\dfrac{x-2\sqrt{xy}+y}{\sqrt{x}-\sqrt{y}}-\dfrac{x-y}{\sqrt{x}+\sqrt{y}}\)

    \(=\dfrac{\left(\sqrt{x}-\sqrt{y}\right)^2}{\sqrt{x}-\sqrt{y}}-\dfrac{\left(\sqrt{x}-\sqrt{y}\right)\left(\sqrt{x}+\sqrt{y}\right)}{\sqrt{x}+\sqrt{y}}\)

    \(=\sqrt{x}-\sqrt{y}-\sqrt{x}+\sqrt{y}=0\)

Ex2:

1.Adam asked me if I liked doing gymnastics

2.Bill asked Sandra if she was coming the following day

3.Bill asked Olga what country you came from

4.Ann asked me if I had seen the library

5.Sammy asked me when I would come and see him again

6.Lisa asked Chris how long you have waited for the doctor

7.Mr.Jones asked if parents could visit the school at any time

8.Nick asked his father if New York is the biggest city in America (câu này diễn tả sự thật hiển nhiên nên ko lùi thì)

9.Dora asked Mike how many students there were in his class

10.Ann asked me what you were going to study

Đặt 2n+1=a²,3n+1=b²(\(a,b\in N;a,b>1\))

Ta có: 4(2n+1)-3n+1=4a²-b²

  <=> 5n+3=(2a+b)(2a-b)

=> 5n+3 là hợp số

Ta có:15^x:5^x=81

<=> 3^x.5^x:5^x=81

<=> 3^x=81=3⁴

<=> x=4

Ta có:\(5^{x-1}\ge5^{x^2-x-9}\)

   \(\Leftrightarrow x-1\ge x^2-x-9\)

   \(\Leftrightarrow x^2-2x-8\le0\)

   \(\Leftrightarrow\left(x-4\right)\left(x+2\right)\le0\)

   \(\Leftrightarrow\left[{}\begin{matrix}\left\{{}\begin{matrix}x-4\le0\\x+2\ge0\end{matrix}\right.\\\left\{{}\begin{matrix}x-4\ge0\\x+2\le0\end{matrix}\right.\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}\left\{{}\begin{matrix}x\le4\\x\ge-2\end{matrix}\right.\\\left\{{}\begin{matrix}x\ge4\\x\le-2\end{matrix}\right.\end{matrix}\right.\Leftrightarrow-2\le x\le4\)

tách \(\overline{mn}=10m+n\)

1.have just cycled

2.cycled

3.wrote

4.have already written

5.have just rung

6.rang

7.watched

8.have already seen

9.spent

10.haven't been

will soon wear

\(n_{CO_2}=\dfrac{52,8}{44}=1,2\left(mol\right);n_{H_2O}=\dfrac{10,8}{18}=0,6\left(mol\right)\)

\(PTHH:2C_2H_2+5O_2\underrightarrow{t^o}4CO_2+2H_2O\)

Mol:             0,6        1,5       1,2          0,6

\(\Rightarrow a=m_{C_2H_2}=0,6.26=15,6\left(g\right)\)

Ta có:\(V_{kk}=5.V_{O_2}=5.1,5.22,4=168\left(l\right)\)