\(A=\frac{1}{1+2}+\frac{1}{1+2+3}+\frac{1}{1+2+3+4}+\frac{1}{1+2+3+4+5}\)

\(A=\frac{1}{\left(1+2\right).2:2}+\frac{1}{\left(1+3\right).3:2}+\frac{1}{\left(1+4\right).4:2}+\frac{1}{\left(1+5\right).5:2}\)

\(A=\frac{2}{2.3}+\frac{2}{3.4}+\frac{2}{4.5}+\frac{2}{5.6}\)

\(A=2\left(\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+\frac{1}{5.6}\right)\)

\(A=2\left(\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}\right)\)

\(A=2\left(\frac{1}{2}-\frac{1}{6}\right)\)

\(A=2.\frac{1}{3}=\frac{2}{3}\)

A= 1/3 + 1/3^2 + ... + 1/3^8

3A= 3. (1/3+ 1/3^2+ ... + 1/3^8)

3A=1+ 1/3 + 1/3^2+ ... +1/3^7

=> 3A - A= (1 + 1/3 + 1/3^2 + ... + 1/3^7) - (1/3 + 1/3^2+ ... + 1/3^8)

=> 2A= 1 - 1/ 3^8

2A= 6560/6561

A= 6560/6561 : 2

A= 3280/6561

tính nhanh 

49/5-1/2-1/4-1/8-1/16-1/32=8,83125

nha bn duyệt đi

\(\frac{49}{5}-\frac{1}{2}-\frac{1}{4}-\frac{1}{8}-\frac{1}{16}-\frac{1}{32}=\frac{1413}{160}=8,83125\)

\(A=1+2\left(\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{98.99}+\frac{1}{99.100}\right)\)

\(A=1+2\left(\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{99}-\frac{1}{100}\right)\)

\(A=1+2\left(\frac{1}{2}-\frac{1}{100}\right)=1+2.\frac{49}{100}=1+\frac{49}{50}\)

\(A=\frac{99}{50}\)

Vậy \(A=\frac{99}{50}\)

A = 1 - 2 + 3 - 4 +...+ 43 - 44 + 45

=> A = (1 - 2) + (3 - 4) +....+ (43 - 44) + 45

=> A = -1 + (-1) +....+ (-1) + 45

=> A = (-1).22 + 45

=> A = -22 + 45

=> A = 23

20 vì :

Khi  - cộng thì 2 số có cùng một tích

Vậy thì ra thôi

20 vì :

Khi trừ cộng thì tích thay đổi luân hoàn 

a: Ta có: \(-\left(-3x^2\right)^3+4x-9-27x^6\)

\(=27x^6-27x^6+4x-9\)

=4x-9

=-1