C. Ta có Truyện sách đánh giặc đúng đắn, linh hoạt, sáng tạo.

Nửa chu vi hình chữ nhật:

80 : 2 = 40 (cm)

Chiều rộng hình chữ nhật:

80 . 1 : 10 = 8 (cm)

Chiều dài hình chữ nhật:

40 - 8 = 32 (cm)

Diện tích hình chữ nhật:

32 . 8 = 256 (cm²)

Đáp số:256cm²

1/(1∗2)+1/(2∗3)+1/(3∗4)+,.,,+1/(n(n+1))

\(= \left(\right. 2 - 1 \left.\right) / \left(\right. 1 * 2 \left.\right) + \left(\right. 3 - 2 \left.\right) / \left(\right. 2 * 3 \left.\right) + \left(\right. 4 - 3 \left.\right) / \left(\right. 3 * 4 \left.\right) + . . . + \left(\right. \left(\right. n + 1 \left.\right) - n \left.\right) / \left(\right. n \left(\right. n + 1 \left.\right) \left.\right)\)

\(= 2 / \left(\right. 1 * 2 \left.\right) - 1 / \left(\right. 1 * 2 \left.\right) + 3 / \left(\right. 2 * 3 \left.\right) - 2 / \left(\right. 2 * 3 \left.\right) + 4 / \left(\right. 3 * 4 \left.\right) - 3 / \left(\right. 3 * 4 \left.\right) + . . . + \left(\right. n + 1 \left.\right) / \left(\right. n \left(\right. n + 1 \left.\right) \left.\right) - n / \left(\right. n \left(\right. n + 1 \left.\right) \left.\right)\)

\(= 1 - 1 / 2 + 1 / 2 - 1 / 3 + 1 / 3 - 1 / 4 + . . . + 1 / n - 1 / \left(\right. n + 1 \left.\right)\)

\(= 1 + \left(\right. 1 / 2 - 1 / 2 \left.\right) + \left(\right. 1 / 3 - 1 / 3 \left.\right) + \left(\right. 1 / 4 - 1 / 4 \left.\right) + . . . + \left(\right. 1 / n - 1 / n \left.\right) + 1 / \left(\right. n + 1 \left.\right)\)

\(= 1 + 0 + 0 + . . . + 0 - 1 / \left(\right. n + 1 \left.\right)\)

\(= 1 - 1 / \left(\right. n + 1 \left.\right)\)

\(= \left(\right. n + 1 \left.\right) / \left(\right. n + 1 \left.\right) - 1 / \left(\right. n + 1 \left.\right)\)

\(= \left(\right. n + 1 - 1 \left.\right) / \left(\right. n + 1 \left.\right)\)

\(= n / \left(\right. n + 1 \left.\right)\)
Vậy: \(1 / \left(\right. 1 * 2 \left.\right) + 1 / \left(\right. 2 * 3 \left.\right) + 1 / \left(\right. 3 * 4 \left.\right) + . . . + 1 / \left(\right. n \left(\right. n + 1 \left.\right) \left.\right) = n / \left(\right. n + 1 \left.\right)\)

các bạn đầu óc phong phú giàu từ thật đấy!

4648473 + 2564267 = 7212740

537385-46278=491107

a. They listen to English every day

(-): They don't listen to English every day.

(?): Do they listen to English every day?

b. She plays badminton every afternoon

(-): She doesn't play badminton every afternoon.

(?): Does she play badminton every afternoon?

c. We play volleyball every morning

(-): We don't play volleyball every morning.

(?): Do we play volleyball every morning?

d. He does homework every night

(-): He doesn't do homework every night.

(?): Does he do homework every night?

. You/ have/ an art lesson on monday (no)

- Do you have an art lesson on Monday?
- No, I don't.

2. They/ ride/ their bikes to school (yes)

- Do they ride their bikes to school?
- Yes, they do.

1. Jane/ play/ tennis at school (no)

- Does Jane play tennis at school?
- No, she doesn't.

2. Your parents/ at home now (yes)

- Are your parents at home now?
- Yes, they are.

3. He/ have/ breakfast at 6 o'clock every day (yes)

- Does he have breakfast at 6 o'clock every day?
- Yes, he does.

3x - 17 = 28

3x = 28 + 17

3x = 45

x = 45 : 3

x = 15

Vậy x = 15