Bài 1:

\(54\left(\dfrac{km}{h}\right)=15\left(\dfrac{m}{s}\right);9\left(\dfrac{m}{s}\right)=32,4\left(\dfrac{km}{h}\right)\)

Baì 2:

\(t'=s':v'=5:\left(5.3,6\right)=\dfrac{5}{18}h\)

\(\Rightarrow v_{tb}=\dfrac{s'+s''}{t'+t''}=\dfrac{5+3,8}{\dfrac{5}{18}+\left(\dfrac{15}{60}\right)}\simeq16,67\left(\dfrac{km}{h}\right)\)

Câu 2: 

\(\Leftrightarrow\left(x+2\right)\left(10x+3\right)=0\)

\(\Leftrightarrow\left[{}\begin{matrix}x=-2\\x=-\dfrac{3}{10}\end{matrix}\right.\)

7B, 8A

Câu 9:

a. <=> 4x= 12

<=> x=3

S={3}

b. <=> (2x-6).(x+9)=0

<=> 2x-6=0 hoặc x+9=0

<=> x= 3     hoặc x=-9

S={3;-9}

c. <=> 5x=-20

<=> x= -4

S={-4}

d. <=> (2x-6).(3x+9)=0

<=> 2x-6=0 hoặc 3x+9=0

<=> 2x=6   hoặc 3x=-9

<=> x=3     hoặc x= -3

S={3;-3}

e. th1: 2x-3= 6x+5 nếu 2x-3>0 => x>\(\dfrac{3}{2}\)

2x-3=6x+5

<=>2x-6x= 5+3

<=>-4x=8

<=> x= -2 (loại)

th2: 2x-3= -6x+5 nếu 2x-3<0 => x<\(\dfrac{3}{2}\)

2x-3=-6x+5

<=>2x+6x= 5+3

<=>8x=8

<=>x=1 (chọn)

S={1}

f. <=> -12x>6

<=> x< -\(\dfrac{1}{2}\)

S={x/x<-\(\dfrac{1}{2}\)}

g. th1: 2x+3=4x+5 nếu 2x+3>0 => x>\(\dfrac{-3}{2}\)

2x+3=4x+5

2x-4x=5-3

-2x= 2

x= -1 (chọn)

th2: 2x+3=-4x+5 nếu 2x+3<0 => x<\(\dfrac{-3}{2}\)

2x+3=-4x+5

2x+4x= 5-3

6x=2

x= \(\dfrac{1}{3}\)(loại)

S={-1}

h. <=> -2x>-6

<=> x< 3

S={x/x<3}

5.

1. going

2.lying/reading

3.likes/ being

4.collecting

5.watching / are going

6.doing

7.plays

8.have collected

9. will travel

10.will make

6.

1.beautiful

2. boring

3. decoration

4.widens

5. wonderful

8

Hình mờ lắm ak bn!

Đặt \(\dfrac{x}{4}=\dfrac{y}{7}=k\)

\(\Leftrightarrow\left\{{}\begin{matrix}x=4k\\y=7k\end{matrix}\right.\)

Ta có: xy=112

\(\Leftrightarrow28k^2=112\)

\(\Leftrightarrow k^2=4\)

Trường hợp 1: k=2

\(\Leftrightarrow\left\{{}\begin{matrix}x=4k=4\cdot2=8\\y=7k=7\cdot2=14\end{matrix}\right.\)

Trường hợp 2: x=-2

\(\Leftrightarrow\left\{{}\begin{matrix}x=4k=-8\\y=7k=-14\end{matrix}\right.\)