Question

tính tổng A=1/2.3+1/3.4+1/4.5+...1/25.26

Answer 1

A= 1/2-1/3+1/3-1/4+1/4-1/5+.....1/25-1/26 A=1/2-1/26 A=6/13

Answer 2

\(A=\frac{1}{2\cdot3}+\frac{1}{3\cdot4}+\frac{1}{4\cdot5}+...+\frac{1}{25\cdot26}\)

\(\Rightarrow A=\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{25}-\frac{1}{26}\)

\(\Rightarrow A=\frac{1}{2}-\frac{1}{26}\)

\(\Rightarrow A=\frac{12}{26}=\frac{6}{13}\)