Question

Tìm A :

A= 2/1*4 + 2/4*7 + 2/7*10 + ... +2/43*46

 

Answer 1

\(A=\frac{2}{1\cdot4}+\frac{2}{4\cdot7}+\frac{2}{7\cdot10}+...+\frac{2}{43\cdot46}\)

\(A=\frac{2}{3}\left(\frac{1}{1\cdot4}+\frac{1}{4\cdot7}+\frac{1}{7\cdot10}+...+\frac{1}{43\cdot46}\right)\)

\(A=\frac{2}{3}\left(1-\frac{1}{4}+\frac{1}{4}-\frac{1}{7}+...+\frac{1}{43}-\frac{1}{46}\right)\)

\(A=\frac{2}{3}\left(1-\frac{1}{46}\right)=\frac{2}{3}\cdot\frac{45}{46}=\frac{15}{23}\)

Answer 2

Cảm ơn bạn Sasuke nhé.