Bài 3:

1, Áp dụng t/c dtsbn:

\(\dfrac{x}{6}=\dfrac{y}{4}=\dfrac{z}{3}=\dfrac{z-x}{3-6}=\dfrac{-21}{-3}=7\\ \Rightarrow\left\{{}\begin{matrix}x=42\\y=28\\z=21\end{matrix}\right.\)

2, Áp dụng t/c dtsbn:

\(\dfrac{x}{3}=\dfrac{y}{5}=\dfrac{z}{7}=\dfrac{2x+3y-z}{6+15-7}=\dfrac{-14}{14}=-1\\ \Rightarrow\left\{{}\begin{matrix}x=-3\\y=-5\\z=-7\end{matrix}\right.\)

Bài 4: 

Áp dụng tính chất của dãy tỉ số bằng nhau, ta được:

\(\dfrac{x}{\dfrac{1}{2}}=\dfrac{y}{\dfrac{1}{3}}=\dfrac{z}{\dfrac{1}{4}}=\dfrac{x+y+z}{\dfrac{1}{2}+\dfrac{1}{3}+\dfrac{1}{4}}=\dfrac{130}{\dfrac{13}{12}}=120\)

Do đó: x=60; y=40; z=30

giúp mk bài 4 với ạ

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Câu 6:

a: =12x^2+4x-3x-1-5x^2+15x-x^2+7x-12

=6x^2+23x-13

b: =5x^2+5x-2x-2-3x^3+3x^2+9x-2x(x^2-9x+20)

=-3x^3+8x^2+14x-2-2x^3+18x^2-40x

=-5x^3+26x^2-26x-2

In my house , I have a my room . It is the best room for me . I love it very much .I can store a lot of my important things in it. I will clean the room from time to time. There was a small bed in my room, which was just enough for me to sleep alone. There's also a small cupboard in the room to put up my clothes. Next to the bed where I slept, there was a white carpet under the small round table. That table was where I used to play chibi dolls. Ah, forgot the bookshelf on the headboard. The bookshelf is neatly lined with thick novels.Seeing me keep the room clean, my parents are very pleased.they kept complimenting me.I find it essential to keep the rooms in order.I love my room and I want the other rooms to be that.

I live in a big city, and my house is not very big since there are only rich people who can afford to live in a spacious house. Every room in my house is just enough for our activities, so we do not have much space for the living room. However, we try our best to decorate it because it is where the whole family gathers after a long day staying outside. We lay down a thick rug on the floor, it both helps to decorate and keep the floor safe from the scratches. To save some space, we put the TV and the loud speakers on the wall. The Play Station, DVD player and other electronic devices are put in a small cabinet in the corner of the room, and we only take them out whenever we want to play something. In the middle of the room is a glass table, we put on the surface a bottle of fresh water and some cups so everyone can have a quick drink before leaving or after coming back to the house. Behind the table is a big white couch, and it can be considered as the biggest thing in the living room. My mother loves the pastel tone, so the walls are painted in cream color; and they all go very well with the couch. We do not have any window there, so we install an air conditioner to keep the atmosphere cool. We hang some pictures of landscape as well as painting on the walls to make the room livelier. There are also a pot of a small plant in the corner of the room, so we can get some fresh air from it. Every night, we would gather to watch TV or sing karaoke, and it is my favorite time of the day. Although the living room is no too big, our family will always have good times there together.

STUDY WELL

6. They would rather Nam didn't smoke in the office.

7. You should practice speaking English every day.

8. What did you talk to him last night?

9. It isn't easy to learn English in a short time.

20. Your room is larger and more comfortable than ours.

6.      They would rather Nam not smoking in the office.

7.      You should practice speaking English every day.

8.      What did you talk to him about last night?

9.      It’s not easy learning English in a short time.

10.  Your room is larger and more comfortable than ours.