không biết giải

2001 

____

1991

A = 2/3 . 3/4 . 4/5 ..... 98/99

A = 2 . 3 . 4 ....... 98/3 . 4 . 5 ...... 99

A = 2/99

A=(1-1/3)x(1-1/4)...x(1-1/99)

  =2/3x3/4x4/5x...x98/99

\(A=\left(1-\frac{1}{3}\right)\cdot\left(1-\frac{1}{4}\right)\cdot\left(1-\frac{1}{5}\right)\cdot...\cdot\left(1-\frac{1}{99}\right)\)

\(A=\frac{2}{3}\cdot\frac{3}{4}\cdot\frac{4}{5}\cdot...\cdot\frac{98}{99}\)

\(A=\frac{2}{99}\)

2/99 nha bạn mình làm rồi

ở violympic đúng ko

\(\left(1-\frac{1}{3}\right)\cdot\left(1-\frac{1}{4}\right)\cdot\left(1-\frac{1}{5}\right)\cdot...\cdot\left(1-\frac{1}{99}\right)\)

\(=\frac{2}{3^{\left(1\right)}}\cdot\frac{3^{\left(1\right)}}{4^{\left(1\right)}}\cdot\frac{4^{\left(1\right)}}{5^{\left(1\right)}}\cdot...\cdot\frac{98^{\left(1\right)}}{99}\)

\(=\frac{2}{99}\)

mình chỉ biết bằng 2/99

1-1/3=2/3; 1-1/4=3/4; 1-1/5=4/5....; 1-1/99=98/99

=> A= (2.3.4.5....98):(3.4.5....99)=2/99

Đs: 2/99

A = 2/99 nhé !

kết  bạn với mình nha mình hết lượt  rồi

a: \(\left(1-\frac12\right)\times\left(1-\frac13\right)\times\ldots\times\left(1-\frac{1}{99}\right)\)

\(=\frac12\times\frac23\times\ldots\times\frac{98}{99}\)

\(=\frac{1}{99}\)

b: \(\left(x+\frac12\right)+\left(x+\frac16\right)+\left(x+\frac{1}{12}\right)+\left(x+\frac{1}{20}\right)+\cdots+\left(x+\frac{1}{90}\right)=\frac{99}{10}\)

=>\(\left(x+\frac{1}{1\times2}\right)+\left(x+\frac{1}{2\times3}\right)+\cdots+\left(x+\frac{1}{9\times10}\right)=\frac{99}{10}\)

=>\(9\times x+1-\frac12+\frac12-\frac13+\cdots+\frac19-\frac{1}{10}=\frac{99}{10}\)

=>\(9\times x+1-\frac{1}{10}=\frac{99}{10}\)

=>\(9\times x=\frac{99}{10}+\frac{1}{10}-1=\frac{100}{10}-1=10-1=9\)

=>x=1

\(1\frac{1}{2}\times1\frac{1}{3}\times1\frac{1}{4}\times...\times1\frac{1}{100}\)

\(=\frac{3}{2}\times\frac{4}{3}\times\frac{5}{4}\times...\times\frac{101}{100}\)

\(=\frac{3\times4\times5\times...\times101}{2\times3\times4\times...\times100}\)

\(=\frac{101}{2}\)