đăng cái này làm gì ?

làm đc k

đáp án là : 35/429

các pn thân iu ơi giúp mik với nha! chìu nay mik cần r!

https://www.youtube.com/watch?v=vNJ2wLqK7Mw

lên đây tham khảo nha

a) 9 + 99 + 999 + ... + 999999

= (10 - 1) + (100 - 1) + (1000 - 1) + ... + (1000000 - 1)

= (10¹ + 10² + 10³ + ... + 10⁶) - (1.6)

= 1111110 - 6 = 1111104

b) 1 + 11 + 111 + ... + 1111111

= 1 + (10¹ + 1) + (10² + 10¹ + 1) + ... + (10⁶ + 10⁵ + 10⁴ + 10³ + 10² + 10¹ + 1)

= 10¹ . 6 + 10² . 5 + 10³ . 4 + ... + 10⁶. 1) + (1 + 1.6)

= 60 + 500 + 4000 + ... + 1000000 + 7

= 1234560 + 7 = 1234567

c) C = 1.2 + 2.3 + 3.4 + 4.5 + ... + 98.99

3C = 1.2.3 + 2.3.3 + 3.4.3 + 4.5.3 + ... + 98.99.3

3C = 1.2.3 + 2.3.(4 - 1) + 3.4.(5 - 2) + ... + 98.99.(100 - 97)

3C = 1.2.3 + 2.3.4 - 2.3.1 + 3.4.5 - 3.4.2 +...+ 98.99.100 - 98.99.97

3C = 98.99.100

C = \(\dfrac{98.99.100}{3}\) = 323400

d) D = 1.3.5 + 3.5.7 + 5.7.9 + ... + 95.97.99

8D = 1.3.5.8 + 3.5.7.8 + 5.7.9.8 + ... + 95.97.99.8

8D = 1.3.5.(7 + 1) + 3.5.7.(9 - 1) + 5.7.9.(11 - 3) + ... + 95.97.99.(101 - 93)

8D = 1.3.5.7 + 1.3.5.1 + 3.5.7.9 - 3.5.7.1 + 5.7.9.11 - 5.7.9.3 + ... + 95.97.99.101 - 95.97.99.93

8D = 1.3.5.1 + 95.97.99.101

D = \(\dfrac{1.3.5.1+95.97.99.101}{8}=15517600\)

(chỉnh đề)

A=\(-1+2-3-4-5+6-7-8-9+...-2021-2022+2023-2024\)

=\(\left(-1-2024\right)+\left(2+2023\right)+\left(-3-2022\right)+\left(-4-2021\right)+\left(-5-2020\right)+\left(6+2019\right)-\left(-7-2018\right)+\left(-8-2017\right)+\left(-9-2016\right)+...+\left(1010+1015\right)+\left(-1011-1014\right)+\left(-1012-1013\right)\)=\(-2025+2025-2025-2025-2025+2025-2025-2025-2025+...+2025-2025-2025\)=253.2025-1771.2025=-3 073 950.

B=\(1.3.5+3.5.7+5.7.9+7.9.11+...+99.101.103\)

8B=\(1.3.5.8+3.5.7.8+5.7.9.8+7.9.11.8+...+99.101.103.8\)

8B=\(1.3.5.\left[7-\left(-1\right)\right]+3.5.7.\left(9-1\right)+5.7.9.\left(11-3\right)+7.9.11.\left(13-5\right)+...+99.101.103.\left(105-97\right)\)8B=\(3.5+3.5.7+3.5.7.9-3.5.7+5.7.9.11-3.5.7.9+7.9.11.13-5.7.9.11+...+99.101.103.105-97.99.101.103\)

B=\(\dfrac{3.5+99.101.103.105}{8}=13517400\)

- Đặt C₁=\(1.3+3.5+...+197.199\).

6C₁=\(1.3.6+3.5.6+...+197.199.6\)

6C₁=\(1.3.\left[5-\left(-1\right)\right]+3.5.\left(7-1\right)+...+197.199.\left(201-195\right)\)

6C₁=\(3+3.5+3.5.7-3.5+...+197.199.201-195.197.199\)
C₁=\(\dfrac{3+197.199.201}{6}=1313301\)

- Đặt C₂=\(2.4+4.6+...+198.200\)

6C₂=\(2.4.6+4.6.6+...+198.200.6\)

6C₂=\(2.4.\left(6-0\right)+4.6.\left(8-2\right)+...+198.200.\left(202-196\right)\)

C₂=\(\dfrac{198.200.202}{6}=1333200\)

=>C₁+C₂=C=1313301+1333200=2646501

 D = 4/1.3.5+4/3.5.7+4/5.7.9+4/7.9.11+4/9.11.13

D.\(=\frac{1}{1.3}-\frac{1}{3.5}+\frac{1}{3.5}-\frac{1}{5.7}+...+\frac{1}{9.11}-\frac{1}{11.13}\)

D\(=\frac{1}{1.3}-\frac{1}{11.13}\)

D\(=\frac{1}{3}-\frac{1}{143}\Rightarrow D=\frac{140}{429}\)

mk ko viết đề nhé

C=1.3.5.8 + 3.5.7.8 + 5.7.9.8 + … + 95.97.99.8

C= 1.3.5(7 + 1) + 3.5.7(9 - 1) + 5.7.9(11 - 3) + … + 95.97.99(101 - 93)

C= 1.3.5.7 + 15 + 3.5.7.9 - 1.3.5.7 + 5.7.9.11 - 3.5.7.9 + … + 95.97.99.101 - 93.95.97.99

C= 15 + 95.97.99.101

=>C=\frac{15 + 95.97.99.101}{8}

1.3.5.8 + 3.5.7.8 + 5.7.9.8 + … + 95.97.99.8

= 1.3.5(7 + 1) + 3.5.7(9 - 1) + 5.7.9(11 - 3) + … + 95.97.99(101 - 93)

= 1.3.5.7 + 15 + 3.5.7.9 - 1.3.5.7 + 5.7.9.11 - 3.5.7.9 + … + 95.97.99.101 - 93.95.97.99

=15 + 95.97.99.101

=> A = \frac{15.95 + 97.99.101}{8}