A = 1/2 + 1/6 + 1/12 + .. + 1/6480
A= 1/1 x 2 + 1/2 x 3 + 1/3 x 4 + ... + 1/80 x 81
A = 1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + ... + 1/80 - 1/81
A = 1 - 1/81
A = 80/81
\(A=\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+...+\frac{1}{6480}\)
\(A=\frac{1}{1}\times2+\frac{1}{2}\times3+\frac{1}{3}\times4+...+\frac{1}{80}\times81\)
\(A=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-...+\frac{1}{80}-\frac{1}{81}\)
\(A=1-\frac{1}{81}\)
\(A=\frac{80}{81}\)
\(A=\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+...+\frac{1}{6084}\)
\(\Rightarrow A=\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{80.81}\)
\(\Rightarrow A=\frac{1}{1}-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{80}-\frac{1}{81}\)
\(\Rightarrow A=1-\frac{1}{81}=\frac{80}{81}\)
A = 1/2 + 1/6 + 1/12 + ... + 1/6480
A = 1/1.2 + 1/2.3 + 1/3.4 + 1/80.81
A = 1/1 - 1/2 + 1/2 - 1/3 + ... + 1/80 - 1/81
A = 1/1 - 1/81
A = 80/81
Vậy A = 80/81
A = 1/2 + 1/6 + 1/12 + ... + 1/6480
A = 1/1.2 + 1/2.3 + 1/3.4 + 1/80.81
A = 1/1 - 1/2 + 1/2 - 1/3 + ... + 1/80 - 1/81
A = 1/1 - 1/81
A = 80/81
Vậy A = 80/81
\(A=\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+...+\frac{1}{6480}\)
\(\Leftrightarrow A=\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+.....+\frac{1}{79.80}+\frac{1}{80.81}\)
\(\Leftrightarrow A=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{4}-\frac{1}{4}+......+\frac{1}{80}-\frac{1}{81}\)
\(\Rightarrow A=1-\frac{1}{81}\)
\(V\text{ậy}A=\frac{80}{81}\)
