41. C → is

42. C → would be

43. C → had known

44. C → will 

45. A → had slept  

Exercise VII:

41. C → is

42. B → to be

43. C → because

45. A → being

Exercise VIII:

46. D

47. C

48. B

49. D

50. A

a. Vì \(R_2//R_3\) nên \(\dfrac{1}{R_{23}}=\dfrac{1}{R_2}+\dfrac{1}{R_3}\Rightarrow R_{23}=\dfrac{R_2.R_3}{R_2+R_3}=\dfrac{30^2}{30+30}=15\left(\text{Ω}\right)\)

Vì \(R_1ntR_{23}\) nên \(R_{td}=R_1+R_{23}=15+15=30\left(\text{Ω}\right)\)

b. Ta có \(I=I_1=I_{23}=\dfrac{U}{R_{td}}=\dfrac{12}{30}=0.4\left(A\right)\)

Mặt khác, \(U_2=U_3\Leftrightarrow I_2.R_2=I_3.R_3\Leftrightarrow30I_2=30.I_3\Leftrightarrow I_2=I_3\) 

\(\Rightarrow I_2=I_3=\dfrac{0,4}{2}=0,2\left(A\right)\)

Theo quy tắc hóa trị

\(ax=by\Rightarrow\dfrac{x}{y}=\dfrac{b}{a}\Rightarrow\dfrac{x}{y}=\dfrac{II}{I}=\dfrac{2}{1}\) \(\Rightarrow\left\{{}\begin{matrix}x=2\\y=1\end{matrix}\right.\)

Vậy CTHH: \(Na_2O\)

\(\)

a. \(\left(2x+1\right)^2=4x^2+4x+1\)

b. \(\left(x-3y\right)\left(x+3y\right)=x^2-9y^2=-\left(x^2+9y^2\right)\)

c. \(\left(5-x\right)^2=25-10x+x^2\)

1. Would you mind doing it for me?

2. She is not used to staying up late

3. The children enjoy going in the rain

4. I suggest going out for a meal tonight

5. She spent 2 hours driving from London to Edinburgh