\(\frac{2}{3\times5}+\frac{2}{5\times7}+\frac{2}{7\times9}+...+\frac{2}{21\times23}\)
\(=\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+...+\frac{1}{21}-\frac{1}{23}\)
\(=\frac{1}{3}-\frac{1}{23}\)
\(=\frac{20}{69}\)
2 / 3 x 5 + 2 / 5 x 7 + 2 / 7 x 9 + ... + 2 / 21 x 23
= 1/3 - 1/5 + 1/5 - 1/7 + 1/7 - 1/9 + ... + 1/21 - 1/23
= 1/3 - 1/13
= 20/69
\(\frac{2}{3.5}+\frac{2}{5.7}+\frac{2}{7.9}+...+\frac{2}{21.23}\)
\(=\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+...+\frac{1}{21}-\frac{1}{23}\)
\(=\frac{1}{3}-\frac{1}{23}\)
\(=\frac{20}{69}\)
Dấu chấm ( . ) ở cấp 2 là dấu nhân ( x ) bn nhé
\(\frac{2}{3\times5}+\frac{2}{5\times7}+\frac{2}{7\times9}+...+\frac{2}{21\times23}\)
\(=\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+...+\frac{1}{21}-\frac{1}{23}\)
\(=\frac{1}{3}-\frac{1}{23}\)
\(=\frac{23}{69}-\frac{3}{69}\)
\(=\frac{20}{69}\)
