Đề sai nhá dãy số lẻ ko thể kết thúc bằng số chẵn đc :
Đề này nhá \(A=\frac{4}{1.3}+\frac{4}{3.5}+\frac{4}{5.7}+.....+\frac{4}{99.101}\)
\(\Rightarrow A=2\left(\frac{2}{1.3}+\frac{2}{3.5}+\frac{2}{5.7}+......+\frac{2}{99.101}\right)\)
\(\Rightarrow A=2\left(1-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+.....+\frac{1}{99}-\frac{1}{101}\right)\)
\(\Rightarrow A=2\left(1-\frac{1}{101}\right)\)
\(\Rightarrow A=2.\frac{100}{101}=\frac{200}{101}\)
\(A=\frac{4}{1.3}+\frac{4}{3.5}+....+\frac{4}{98.100}\)
\(A=2.\left(\frac{2}{1.3}+\frac{2}{3.5}+.........+\frac{2}{98.100}\right)\)
\(A=2.\left(1-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+.....+\frac{1}{98}-\frac{1}{100}\right)\)
\(A=2.\left(1-\frac{1}{100}\right)\)
\(A=2.\frac{99}{100}\)
\(A=\frac{99}{50}\)
