Sau 2 phút = \(\dfrac{1}{30}\) giờ thì máy bay bay đc \(\dfrac{1}{30}\cdot300=10\left(km\right)\)

Do đó máy bay ở độ cao \(10\cdot\sin25^0\approx4\left(km\right)=4000\left(m\right)\)

IV. Choose the underlined part that needs correction

1 D (impress $\rightarrow$ to impress)

2 C (so that $\rightarrow$ so as to / in order to)

3 C (don't tell $\rightarrow$ won't tell)

4 A (don't have to $\rightarrow$ mustn't)

5 B (quietly $\rightarrow$ quiet)

V. Put the sentences into the correct order to make a complete conversation

1 F (What can I do for you?)

2 D (Could you give me some information, please?)

3 B (I'd like to visit a market. Would you mind suggesting one?)

4 G (I suggest going to the Stamps and Coins Market.)

5 E (That sounds interesting. What time does it open?)

6 A (It opens from 10 am to 6 pm.)

7 C (Thank you very much.)

8 H (You're welcome.)

Cách khác 

\(A=\frac{4x-1}{x^2+3}=\frac{3\left(4x-1\right)}{3\left(x^2+3\right)}=\frac{\left(4x^2+12x+9\right)-4x^2-12}{3\left(x^2+3\right)}=\frac{\left(2x+3\right)^2}{3\left(x^2+3\right)}+\frac{-4\left(x^2+3\right)}{3\left(x^2+3\right)}\)

 \(A=\frac{\left(2x+3\right)^2}{3\left(x^2+3\right)}+\frac{-4}{3}\ge-\frac{4}{3}\)

Vậy Min = -4/3 <=> x = -3/2 

Đặt \(A=\frac{4x-1}{x^2+3}=t\)\(\Rightarrow x^2.t+3t=4x-1\) 

<=> \(x^2.t-4x+3t+1=0\)

Đa thức trên có nghiệm <=> \(\Delta\ge0\)

<=> \(16-4t\left(1+3t\right)\ge0\)

<=> \(16-4t-12t^2\ge0\)

<=> \(3t^2+t-3\le0\)

<=> \(\left(t-1\right)\left(3t+4\right)\le0\)

<=> \(\hept{\begin{cases}t\le1\\t\ge-\frac{4}{3}\end{cases}}\)

Vậy min A = \(-\frac{4}{3}\) <=> \(x=-\frac{3}{2}\)

1 If I were you I would recycle these plastic carrier bags

2 The pilots suddenly struck, so all flights had to be cancelled

3 The polluted environment causes the death of birds and plants

4 The contaminated food leads to people's poor health

Ví dụ : khớp tịnh tiến; khớp quay; khớp cầu ; khớp vít ; khớp cácđăng….. CHÚC BẠN HỌC TỐT!

Vd:

Khớp tịnh tiến, Khớp quay, khớp vít..

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