pn ghi đề đi vì mk k có sách

1 thùng có số quyển vở là:

     2135 : 7 = 305 (quyển)

5 thùng có số quyển vở là:

     305 x 5 = 1525 (quyển)

                    Đáp số: 1525 quyển vở

1525 quyển vở

1 thùng có số quyển vở là :

    2135 : 7 = 305 ( quyển )

5 thùng có số quyển vở là :

    305 x 5 = 1525 ( quyển )

                 Đáp số : 1225 quyển vở.

=>S=2(1-1/3+1/3-1/4+....................-1/2020)

=>S=2*(1-1/2020)

=>s=2* 2019/2020

=>S=2019/1010

\(\frac{2}{1\cdot3}+\frac{2}{3\cdot5}+\frac{2}{5\cdot7}+...+\frac{2}{2017\cdot2019}+\frac{2}{2019\cdot2021}\)

\(=1-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+...+\frac{1}{2017}-\frac{1}{2019}+\frac{1}{2019}-\frac{1}{2021}\)

\(=1-\frac{1}{2021}=\frac{2020}{2021}\)

\(\frac{2}{1\times3}+\frac{2}{3\times5}+\frac{2}{5\times7}+...+\frac{2}{2019\times2021}\)

\(=1-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+...+\frac{1}{2019}-\frac{1}{2021}\)

\(=1-\frac{1}{2021}\)

\(=\frac{2020}{2021}\)

Nowadays almost everyone in the world have had a contact with computers. A great part of them has been using international computer’s net known as Internet.
I think using internet has more advantages then disadvantages. First of all, Internet provides access to a lot of information. Some of them are very useful in your job other helps in your hobby. Searching the net with Google, you can find everything you want. You can also do shopping using Internet. Of course it has no sense to buying a roll, but if you need something inaccessible, you will be able to buy it in Internet shop. You need only to select what you want, fill in some forms and click ok. In a few days you will get what you have ordered directly to your home, without going anywhere. You can pay with your credit card, pay postman when you get the package, or transfer money from your bank account. Nowadays almost every bigger bank offers transferring money with Internet. It is more faster then going to the bank and filling blankets. Transferring money in traditional way takes about a day, using Internet you can do this in few seconds even in Saturday and Sunday evenings. What’s more internet banks give you insurance against unauthorized transaction. Some people thinks that their money can by easily stolen by hackers. It isn’t true because the easiest way to steal your money from internet bank is breaking to your house and stealing you card of codes. Internet banks have very good protections witch make them pracally unbreakable.
Next advantage of internet is email. Internet mails get in a few seconds in their destinations. It is very important when you want to send something for example to Australia. Of course you can send not only texts. You can email whatever you want, movies, photos, songs, computer programs etc. Costs are very important advantage of emails. Sending an email is much cheaper then phoning, or sending normal letters.
Beside a lot of advantages, Internet has some disadvantages too. First of disadvantages is that people who spend too much time sitting in the front of computer can easily gets ill. Radiation emitted by computer’s screen is harmful to eyes. People who spend too much time at their homes (because they needn’t go anywhere, they can do everything using Internet) are getting weaker. Sitting a for long time is also harmful to the spine. 
Second disadvantage is that Internet can by addictive. Some people just can’t live without it. They have no real friends and when Internet is down they are getting furious.
Internet has some opponents but more and more people treats Internet telephone, or radio. They use it for fun and work, and I think at present living without internet would be quite difficult. 

kiểm tra tự làm đi

anh rất tiếc lần sau hỏi che chữ kiểm tra đi nhé

\(2/\\ Fe_2O_3 + 3H_2SO_4 \to Fe_2(SO_4)_3 + 3H_2O\\ Al_2O_3 + 3H_2SO_4 \to Al_2(SO_4)_3 + 3H_2O\\ MgO + H_2SO_4 \to MgSO_4 + H_2O\\ Na_2O + H_2SO_4 \to Na_2SO_4 + H_2O\\ n_{H_2O} = n_{H_2SO_4} = \dfrac{41,16}{98}=0,42(mol)\\ m_{hh} + m_{axit} = m_{Muối} + m_{H_2O} \\ \Rightarrow m_{muối} = 20,66 + 41,16 -0,42.18 = 54,26(gam)\)

\(1/\\ S + O_2 \xrightarrow{t^o} SO_2\\ 2SO_ 2+ O_2 \xrightarrow{t^o,V_2O_5} 2SO_3\\ SO_3 + H_2O \to H_2SO_4\\ n_{H_2SO_4} = n_S = \dfrac{1,28}{32} =0,04(mol)\\ \Rightarrow m_{H_2SO_4} = 0,04.98 = 3,92(gam)\)

\(3/\\ Fe_xO_y + yH_2 \xrightarrow{t^o} xFe + yH_2O\\ ZnO + H_2 \xrightarrow{t^o} Zn + H_2O\\ n_{H_2} = n_{H_2O} = a(mol)\\ \Rightarrow 19,4 + 2a = 15,24 + 18a\\ \Rightarrow a = 0,26\\ \Rightarrow b = 0,26.18 = 4,68(gam)\)

a) Ta có: \(\left(2\sqrt{3}+\sqrt{5}\right)\cdot\sqrt{3}-\sqrt{60}\)

\(=6+\sqrt{15}-2\sqrt{15}\)

\(=6-\sqrt{15}\)

b) Ta có: \(\left(5\sqrt{2}+2\sqrt{5}\right)\cdot\sqrt{5}-\sqrt{250}\)

\(=5\sqrt{10}+10-5\sqrt{10}\)

=10

c) Ta có: \(\left(\sqrt{28}-\sqrt{12}-\sqrt{7}\right)\cdot\sqrt{7}+2\sqrt{21}\)

\(=14-2\sqrt{1}-7+2\sqrt{21}\)

7

4/5 + 3/4 : X = 1/2

3/4:x = 1/2 - 4/5 

3/4 : x = -3/10 

x=3/4 : (-3/10) 

x=-5/2

\(\frac{4}{5}+\frac{3}{4}:x=\frac{1}{2}\)

\(\frac{3}{4}:x=\frac{1}{2}-\frac{4}{5}\)

\(\frac{3}{4}:x=-\frac{3}{10}\)

\(x=\frac{3}{4}:\left(-\frac{3}{10}\right)\)

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

Lớp 5 chưa học số âm, cậu học hè chương trình lớp 6 ak?

\(\frac{4}{5}+\frac{3}{4}:x=\frac{1}{2}\) => \(\frac{3}{4}:x\)=\(\frac{5}{10}\)-\(\frac{8}{10}\)=\(\frac{-3}{10}\)

                                      => \(x=\frac{3}{4}:\frac{-3}{10}=\frac{3}{4}.\frac{10}{-3}=\frac{-5}{2}\)