1. should ask

*must not ask là sai vì người quản lí mới nên hỏi vài câu hỏi trước khi thực hiện thay đổi

*have to ask (mang tính chủ quan) nên có làm hay không làm cũng được nên ta không chọn đáp án này

2. could prevent

Vì sau would, could là V_inf

\(a,B=\dfrac{2+3}{2.2+3}=\dfrac{5}{7}\\ b,A=\dfrac{\sqrt{x}+15-x-3\sqrt{x}+2x-\sqrt{x}-15}{\left(\sqrt{x}-3\right)\left(\sqrt{x}+3\right)}\\ A=\dfrac{x-3\sqrt{x}}{\left(\sqrt{x}-3\right)\left(\sqrt{x}+3\right)}=\dfrac{\sqrt{x}\left(\sqrt{x}-3\right)}{\left(\sqrt{x}-3\right)\left(\sqrt{x}+3\right)}=\dfrac{\sqrt{x}}{\sqrt{x}+3}\\ c,P=AB=\dfrac{\sqrt{x}}{2\sqrt{x}-3}< \dfrac{1}{2}\Leftrightarrow\dfrac{\sqrt{x}}{2\sqrt{x}-3}-\dfrac{1}{2}< 0\\ \Leftrightarrow\dfrac{2\sqrt{x}-2\sqrt{x}+3}{2\left(2\sqrt{x}-3\right)}< 0\Leftrightarrow\dfrac{3}{2\left(2\sqrt{x}-3\right)}< 0\\ \Leftrightarrow2\sqrt{x}-3< 0\left(3>0\right)\\ \Leftrightarrow\sqrt{x}< \dfrac{3}{2}\Leftrightarrow0< x< \dfrac{9}{4}\)

15 They said they would go to the beach if the weather was nice the day after (loại 1)

16 I told Mike Halen would know what to do if she were late (2)

17 Mark said he would have spoken to Frank if he had seen him the day before (3)

18 The girl said he wouldn't run away if she saw a spider (2)

19 David told her if she had too much to do, she could ask him to help her (2)

20 He told me if I studied harder, I would get high marks in next exam (1)

7 Tim told me if I didn't have a degree, I couldn't get a job easily

8 I told Martin he could sleep well at night, if he didn't drink too much coffee

9 Jim told Daise unless she chooses the words more carefully, she would be introuble

10 I said to Dick I would forget Ann's birthday if he didn't remind me

11 Jim told Jane he wouldn't have been late for his intervire if he hadn't missed the train

12 I told my friends if it didn't rain that morning, we could go for a picnic

13 I said if Linda had been at home the day before, I could have visited her

14 They said if the weather were nice that day, they would go to the zoo

\(\dfrac{13}{2}\) : 4\(\dfrac{2}{3}\): 2

= \(\dfrac{13}{2}\): \(\dfrac{14}{3}\):2

= \(\dfrac{13}{2}\) \(\times\) \(\dfrac{3}{14}\):2

=  \(\dfrac{39}{28}\) : 2

= \(\dfrac{39}{28}\) \(\times\) \(\dfrac{1}{2}\)

= \(\dfrac{39}{56}\)

2) 

Đổi 1h15 phút thành 1,25 h

Thời gian dự định là: $\frac{AB}{40}$ (h)

Thời gian thực tế: $\frac{AB}{40-15}=\frac{AB}{25}$ (h)

Chênh lệch thời gian dự định và thời gian thực tế là:

$\frac{AB}{25}-\frac{AB}{40}=1,25$

$\frac{3AB}{200}=1,25\Rightarrow AB=83,33$ (km)

Câu 3:

Đổi 20 phút thành $\frac{1}{3}$ giờ

Giả sử sau khi ô tô đi được $a$ giờ thì hai xe gặp nhau tại $C$. Lúc này, xe máy đã đi được $a+\frac{1}{3}$ giờ

Ta có:

$AC=35(a+\frac{1}{3})=(35+20).a$

$\Leftrightarrow 35(a+\frac{1}{3})=55a$

$\Rightarrow a=\frac{7}{12}$ (h) 

Đổi $\frac{7}{12}$ h = 35 phút. Vậy sau khi đi được 35 phút thì ô tô gặp xe máy.