4 cái kẹo

A = (1 -1/2).( 1 -1/3).(1-1/4)...(1-1/20)

A = 1/2.2/3.3/4...19/20

A = 1.2.3....19/2.3.4...20

A = 1/20

\(\frac{2}{3}+\frac{2}{6}+\frac{2}{12}+\frac{2}{24}+\frac{2}{48}+\frac{29}{96}+\frac{2}{192}\)

\(=\frac{2}{3}+\frac{1}{6}+\frac{1}{12}+\frac{1}{24}+\frac{1}{48}+\frac{1}{96}\)

\(=\frac{254}{192}=\frac{127}{96}\)