Đặt \(A=\frac{9}{13.31}+\frac{8}{31.47}+\frac{7}{47.61}+\frac{2}{61.65}\Rightarrow2A=\frac{18}{13.31}+\frac{16}{31.47}+\frac{14}{47.61}+\frac{4}{61.65}\)
\(=\frac{1}{13}-\frac{1}{31}+\frac{1}{31}-\frac{1}{47}+\frac{1}{47}-\frac{1}{61}+\frac{1}{61}-\frac{1}{65}=\frac{1}{13}-\frac{1}{65}=\frac{4}{65}\)
=>A=\(\frac{4}{65}:2=\frac{2}{65}\)
toán dễ ko chấp , Đặt tổng =A nhân A cho 2 /...................