Ax1/2=1/2+1/6+1/12+1/20+1/30+1/42
Ax1/2=1/1.2+1/2.3+1/3.4+...+1/6.7
Ax1/2=1-1/2+1/2-1/3+1/3-1/4+....+1/6-1/7
Ax1/2=1-1/7=6/7
=>A=6/7x2=12/7
A= \(\frac{1}{4}\)+ \(\frac{1}{12}\)+ \(\frac{1}{24}\)+ \(\frac{1}{40}\)+ \(\frac{1}{60}\)+ \(\frac{1}{84}\)
2A=\(\frac{1}{2}\)+ \(\frac{1}{6}\)+ \(\frac{1}{12}\)+\(\frac{1}{20}\)+ \(\frac{1}{30}\)+ \(\frac{1}{42}\)
2A=\(\frac{1}{1.2}\)+ \(\frac{1}{2.3}\)+ \(\frac{1}{3.4}\)+ \(\frac{1}{4.5}\)+ \(\frac{1}{5.6}\)+ \(\frac{1}{6.7}\)
2A=\(1-\frac{1}{2}\)\(+\frac{1}{2}\)\(-\frac{1}{3}\)\(+...+\)\(\frac{1}{6}\)\(-\frac{1}{7}\)
2A=\(1-\frac{1}{7}\)
2A=\(\frac{6}{7}\)
A=\(\frac{6}{7}:2\)
\(\Rightarrow A=\frac{3}{7}\)
Vậy \(A=\frac{3}{7}\)
Chúc bạn học tốt nhé!
