\(a,(2x−1)^3−(2x+3)^3+48x(x+1)−2019 \)

\(=8x^3-12x^2+6x-1-\left(8x^3+36x^2+54x+27\right)+48x^2+48x-2019\)

\(=8x^3-12x^2+6x-1-8x^3-36x^2-54x-27+48x^2+48x-2019\)

\(=-2047\)

\(b,\) \(( 7 x + 3 ) ^2 − ( 2 x + 11 ) ^2 − x ( 45 x − 2 ) − 17\)

\(=\left(7x+3-2x-11\right)\left(7x+3+2x+11\right)-45x^2+2x-17\)

\(=\left(5x-8\right)\left(9x+14\right)-45x^2+2x-17\)

\(=45x^2+70x-72x-112-45x^2+2x-17\)

\(=-129\)

32. have already shown

33. read - have you read?

34. was walking - felt - didn't know 

\(P=\dfrac{10}{\left(x+2\right)\left(3-x\right)}-\dfrac{12}{\left(3-x\right)\left(3+x\right)}-\dfrac{1}{\left(x+3\right)\left(x+2\right)}\)

   Thay \(x=-0,75\) vào biểu thức P ta có,

\(P=\dfrac{10}{\left(-0,75+2\right)\left(3+0,75\right)}-\dfrac{12}{\left(3+0,75\right)\left(3-0,75\right)}-\dfrac{1}{\left(-0,75+3\right)\left(-0,75+2\right)}\)

    \(=\dfrac{32}{15}-\dfrac{64}{45}-\dfrac{16}{45}\)

    \(=\dfrac{96}{45}-\dfrac{64}{45}-\dfrac{16}{45}\)

    \(=\dfrac{16}{45}\)

Ta có: \(a^2+b^2=20\)

           \(ab=8\)

     \(\left(a+b\right)^2=a^2+2ab+b^2\)

                   \(=2ab+20\)

                   \(=2.8+20\)

                   \(=36\)

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

                   \(=-2ab+20\)

                   \(=-2.8+20\)

                   \(=4\)

    \(\left(a+b\right)^2=36\Rightarrow a+b=6\)

    \(\left(a-b\right)^2=4\Rightarrow a-b=2\)

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

             \(=2.6\)

             \(=12\)    

a)     \(Zn+2HCl\rightarrow ZnCl_2+H_2\uparrow\)

 TPT: 1                                     1     (mol)

 TĐB: 0,1                                 0,1   (mol)

\(n_{Zn}=\dfrac{m}{M}=\dfrac{6,5}{65}=0,1\left(mol\right)\)

b) 

\(V_{H_2}=n.22,4=0,1.22,4=2,24\left(l\right)\)

\(M_{R_xO_y}=d_{R_xO_y}.M_{N_2}=2,286.28=64\) (g/mol)

Mặt khác ta có: \(Rx=16y\)

                  \(\Leftrightarrow Rx+16y=64\)

                  \(\Leftrightarrow16y+16y=64\)

                  \(\Rightarrow y=2\)

      \(Rx+16y=64\)

\(\Leftrightarrow Rx+32=64\)

\(\Leftrightarrow Rx=32\)

       x=1\(\rightarrow R=32\) (g/mol)

Vậy CTHH là \(SO_2\)

bạn xem lại cách làm nhe ra 0,3 là lạ rồi á

sửa câu 1: A letter has been written by Nam for a month 

46. I told Mary not to leave the window open.

47. She said that she would have a cup of tea with me.

48. She said that she would pay him if she could.

49. She asked us what we were going to do the following summer.

50. He told Jack that he would call him the following day.

51. He told us not to keep the door locked.

52. I asked him how long he was going to stay.

53. She wanted to know if I was going by train.

54. She asked us not to use too much hot water.

55. She invited me to come to her party.

56. They wondered how to use that support service.

47. He asked who he should turn to for help.