B=1/2.2/3.3/4.4/5.....19/20
= 1/20
Nha bạn
\(B=\left(1-\frac{1}{2}\right).\left(1-\frac{1}{3}\right).\left(1-\frac{1}{4}\right)....\left(1-\frac{1}{20}\right)\)
\(B=\frac{1}{2}.\frac{2}{3}.\frac{3}{4}....\frac{19}{20}\)
\(B=\frac{1.2.3.....19}{2.3.4...20}=\frac{1.\left(2.3.4....19\right)}{\left(2.3.4.....19\right).20}=\frac{1}{20}\)
\(B=\left(1-\frac{1}{2}\right)\cdot\left(1-\frac{1}{3}\right)\cdot\left(1-\frac{1}{4}\right)\cdot...\cdot\left(1-\frac{1}{20}\right)\)
\(B=\frac{1}{2}\cdot\frac{2}{3}\cdot\frac{3}{4}\cdot...\cdot\frac{19}{20}\)
\(B=\frac{1}{20}\)