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\(B=\frac{1}{2}.\frac{2}{3}.\frac{3}{4}.....\frac{19}{20}\)
\(B=\frac{1}{20}\)
\(B=\left(1-\frac{1}{2}\right).\left(1-\frac{1}{3}\right).....\left(1-\frac{1}{20}\right)\)
\(B=\frac{1}{2}.\frac{2}{3}.....\frac{19}{20}\)
\(B=\frac{1}{20}\)
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 = \(\frac{1}{20}\)
Vậy B = \(\frac{1}{20}\)
\(B=\frac{3}{2}.\frac{4}{3}.\frac{5}{4}......\frac{21}{20}\)
\(B=\frac{21}{2}\)
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\(B=\left(1+\frac{1}{2}\right)\left(1+\frac{1}{3}\right)\left(1+\frac{1}{4}\right)...\left(1+\frac{1}{20}\right)\)
\(\Rightarrow B=\left(\frac{2}{2}+\frac{1}{2}\right)\left(\frac{3}{3}+\frac{1}{3}\right)\left(\frac{4}{4}+\frac{1}{4}\right)...\left(\frac{20}{20}+\frac{1}{20}\right)\)
\(\Rightarrow B=\frac{3}{2}.\frac{4}{3}.\frac{5}{4}...\frac{21}{20}\)
\(\Rightarrow B=\frac{21}{2}\)
B = \(\left(1-\frac{1}{2}\right).\left(1-\frac{1}{3}\right).\left(1-\frac{1}{4}\right)...\left(1-\frac{1}{20}\right)=\frac{1}{2}.\frac{2}{3}.\frac{3}{4}...\frac{19}{20}\)
=\(\frac{1.2.3...19}{2.3.4...20}=\frac{1}{20}\)
B=(1-1/2)(1-1/3).....(1-1/20)
B=1/2.2/3....19/20
B=1-1/20
B=19/20