a) \(\dfrac{-3}{21}+\dfrac{-6}{21}+\dfrac{7}{21}=\dfrac{-2}{21}\) 

b) \(\dfrac{-78}{90}+\dfrac{-25}{90}+\dfrac{40}{90}=\dfrac{-7}{10}\)

c) \(\dfrac{-2}{5}+\dfrac{3}{11}=\dfrac{-22}{55}+\dfrac{15}{55}=\dfrac{-7}{55}\)

d) \(-4+\dfrac{4}{5}-\dfrac{2}{3}=\dfrac{-16}{5}-\dfrac{2}{3}=\dfrac{-48}{15}-\dfrac{10}{15}=\dfrac{58}{15}\)

e) \(\dfrac{3}{5}-\dfrac{4}{3}+\dfrac{3}{4}=\dfrac{36}{60}-\dfrac{80}{60}+\dfrac{45}{60}=\dfrac{1}{60}\)

g) \(\dfrac{5}{8}+\dfrac{2}{5}-\dfrac{3}{10}=\dfrac{25}{40}+\dfrac{16}{40}-\dfrac{12}{40}=\dfrac{29}{40}\)

h) \(\dfrac{3}{4}+\dfrac{1}{12}+\dfrac{5}{3}+\dfrac{2}{9}=\dfrac{5}{6}+\dfrac{17}{9}=\dfrac{49}{18}\)

Phần tử là những đối tượng cụ thể được chứa trong tập hợp.

Nửa chu vi mảnh đất hình chữ nhật là:

90 : 2 = 45 (m)

Chiều rộng mảnh đất hình chữ nhật là:

45 : (2 + 3) x 2 = 18 (m)

Chiều dài mảnh đất hình chữ nhật là:

45 - 18 = 27 (m)

Diện tích mảnh đất hình chữ nhật là:

18 x 27 = 486 (m²)

Diện tích trồng rau là:

486 x 20% = 97,2 (m²)

Diện tích trồng hoa là:

486 : 9 x 2 = 108 (m²)

Diện tích trồng cây ăn quả là:

486 - 97,2 - 108 = 280,8 (m²)

Đáp số: 97,2m² trồng rau

              108m² trồng hoa

               208,8m² trồng cây ăn quả

B. windows

Q. Do you have many friends?

- Yes, i have a tons of friends, thanks to my great social skills! Also because it's also fun to get around with everyone.

- No, i don't, I'm not really a fan of socializing, plus, I'm shy and introverted so I only have a few friends around.

Q. How often do you go out with friends?

- I often go out with my friends so that we're able to get to know each other more and have some conversations together along the way.

- I don't often go out with friends, I'm always busy, by the way, I would rather stay with my family.

Q. Tell me about your best friend at school.

- My best friend has a nice and bubbly personality. He appears with dark brown hair and large, expressive eyes. He finds himself interested in many fleids such as science and art, he also loves doing researches and create crafts in his free time. He's defintely the best person I've ever known!

Q. How friendly are your neighbours?

- My neighbours? They're the best, they are literally the sweetest and friendliest people on earth, they're always ready to help and are eager to give me tons of presents during my brithday.

- No, my neighbours aren't friendly at all, I don't like how they shout at me whenever I cross by! Another thing that bothers me is that these people are really short-tempered and I'm super scared.

Q. Which is more important to you, friends or family?

- They're equally important to me! They all give me comfort, support, motivation for me to keep up. Meanwhile while I'm more expressive with friends, my family makes me feels more warm and close due to the time we have been spending together.

Q. Who do you spend most time studying/working with?

- I enjoy studying with my friends a lot! I always have my smart dudes to help with my problems while giving really nice advices. Doing stuffs with them is so fun, too, we learn hard yet play hard.

Q. What kinds of things do you study/work with other people?

- I love studying psycology with other people, not just because It's about human behaviors that I could test on humans but also because I have a lot of interest in the field.

Q. Are there times when you study/work better by yourself?

- I have to agree that when I'm by myself, I'm able to focus and concentrate way better, meanwhile, the peaceful, quiet energy helps me a lot with motivation.

- Absolutely not, It's so boring to study alone, I also need the others to teach and guide me so that I wouldn't get into the wrong way.

Q. Is it important to like the person you study/work with?

- Of course, because we would work together more effectively and so that we would have less troubles.

- Well, not really, I think. We should all adapt and manage troubles with hard people because society is not small, there's a high chance we"ll be facing even more difficult people so there's no need to like the person that you work or study with.

132 = 44 x 3 = 12 x 11 = 33 x 4 = 66 x 2 = 132 x 1

Tổng quát: \(\dfrac{3}{\left(1.2\right)^2}+\dfrac{5}{\left(2.3\right)^2}+...+\dfrac{2n+1}{\left[n\left(n+1\right)\right]}=\dfrac{n\left(n+2\right)}{\left(n+1\right)^2}\)

\(\Rightarrow C=\dfrac{9\left(9+2\right)}{\left(9+1\right)^2}=\dfrac{9.11}{10^2}=\dfrac{99}{100}\)

Vậy \(C=\dfrac{99}{100}\)

59 + 3(x - 8) = 53

3(x - 8) = 53 - 59

3(x - 8) = -6

x - 8 = -6 : 3

x - 8 = -2

x = -2 + 8

x = 6

Vậy x = 6

Ta có:

2! = 1.2

3! = 2.3

4! > 3.4

...

100! > 99.100

⇒ \(D< \dfrac{1}{1.2}+\dfrac{1}{2.3}+\dfrac{1}{3.4}+...+\dfrac{1}{99.100}\)

\(D< 1-\dfrac{1}{100}\)

\(D< \dfrac{99}{100}\)

Mà \(\dfrac{99}{100}< 1\) ⇒ \(D< 1\)

\(\dfrac{18}{117}\times\dfrac{12}{113}+\dfrac{12}{113}\times\dfrac{8}{177}+\dfrac{26}{117}\times\dfrac{101}{113}\)

\(=\left(\dfrac{18}{177}+\dfrac{8}{177}\right)\times\dfrac{12}{113}+\dfrac{26}{117}\times\dfrac{101}{113}\)

\(=\dfrac{26}{177}\times\dfrac{12}{113}+\dfrac{26}{117}\times\dfrac{101}{113}\)

\(=\dfrac{26}{177}\times\left(\dfrac{12}{113}+\dfrac{101}{113}\right)\)

\(=\dfrac{26}{177}\times1\)

\(=\dfrac{26}{117}\)