Lời giải:

Gọi tổng trên là $A$. Ta có:

$A=\frac{(x+2)-(x+1)}{(x+1)(x+2)}+\frac{(x+3)-(x+2)}{(x+2)(x+3)}+\frac{(x+4)-(x+3)}{(x+3)(x+4)}+\frac{(x+5)-(x+4)}{(x+4)(x+5)}$

$=\frac{1}{x+1}-\frac{1}{x+2}+\frac{1}{x+2}-\frac{1}{x+3}+\frac{1}{x+3}-\frac{1}{x+4}+\frac{1}{x+4}-\frac{1}{x+5}$

$=\frac{1}{x+1}-\frac{1}{x+5}=\frac{4}{(x+1)(x+5)}$

3. She said I should ask a lawyer.

4. Mrs Linh asked me to give Tuan this book.

VII

1.It causes ill health in human being.

2.Doctors have proved that air pollution causes lung diseases.

3.Yes,there is. Because gas from cars,petrol produces,... cause air pollution in the city.

4.They can't think as quickly as other children,and are clumsy when they use their hands.

a) 

\(m_H=\dfrac{2,04.98}{100}=2\left(g\right)=>n_H=\dfrac{2}{1}=2\left(mol\right)\)

\(m_S=\dfrac{32,65.98}{100}=32\left(g\right)\) => \(n_S=\dfrac{32}{32}=1\left(mol\right)\)

\(m_O=\dfrac{65,31.98}{100}=64\left(g\right)=>n_O=\dfrac{64}{16}=4\left(mol\right)\)

=> CTHH: H₂SO₄

b) 

n_H = 2.2 = 4(mol)

n_S = 1.2 = 2(mol)

n_O = 4.2 = 8 (mol)

5.D

7.A