Đặt biểu thức trên là $A$ Ta có: $A = \frac{1}{10} + \frac {4}{20} + \frac{9}{30} + \frac{16}{40} + \frac{25}{50} + \frac{36}{60} + \frac{49}{70} + \frac{64}{80} + \frac{81}{90}$ $A = \frac{1}{10} + \frac{2}{10} + \frac{3}{10} + \frac{4}{10} + \frac{5}{10} + \frac{6}{10} + \frac{7}{10} + \frac{8}{10} + \frac{9}{10}$ $A = (\frac{1}{10} + \frac{9}{10}) + (\frac{2}{10} + \frac{8}{10}) + (\frac{3}{10} + \frac{7}{10}) + (\frac{4}{10} + \frac{6}{10}) + \frac {5}{10}$ $A = 1 + 1 + 1 + 1 + \frac {5}{10}$ $A = 1 \times 4 + \frac {1}{2}$ $A = 4 + \frac{1}{2}$ $A = \frac {9}{2}$ Câu trả lời: Đặt biểu thức trên là $A$ Ta có: $A = \frac{1}{10} + \frac {4}{20} + \frac{9}{30} + \frac{16}{40} + \frac{25}{50} + \frac{36}{60} + \frac{49}{70} + \frac{64}{80} + \frac{81}{90}$ $A = \frac{1}{10} + \frac{2}{10} + \frac{3}{10} + \frac{4}{10} + \frac{5}{10} + \frac{6}{10} + \frac{7}{10} + \frac{8}{10} + \frac{9}{10}$ $A = (\frac{1}{10} + \frac{9}{10}) + (\frac{2}{10} + \frac{8}{10}) + (\frac{3}{10} + \frac{7}{10}) + (\frac{4}{10} + \frac{6}{10}) + \frac {5}{10}$ $A = 1 + 1 + 1 + 1 + \frac {5}{10}$ $A = 1 \times 4 + \frac {1}{2}$ $A = 4 + \frac{1}{2}$ $A = \frac {9}{2}$