Câu 18: \(A=\left(1-\frac12\right)\left(1-\frac13\right)\cdot\ldots\cdot\left(1-\frac{1}{20}\right)\)
\(=\frac12\cdot\frac23\cdot\ldots\cdot\frac{19}{20}=\frac{1}{20}\)
=>Chọn A
Cau 19: \(B=\left(1-\frac14\right)\left(1-\frac19\right)\cdot\ldots\cdot\left(1-\frac{1}{100}\right)\)
\(=\left(1-\frac12\right)\left(1-\frac13\right)\cdot\ldots\cdot\left(1-\frac{1}{10}\right)\left(1+\frac12\right)\left(1+\frac13\right)\cdot\ldots\cdot\left(1+\frac{1}{10}\right)\)
\(=\frac12\cdot\frac23\cdot\ldots\cdot\frac{9}{10}\cdot\frac32\cdot\frac43\cdot\ldots\cdot\frac{11}{10}=\frac{1}{10}\cdot\frac{11}{2}=\frac{11}{20}\)
=>Chọn D
