A = 1/5×10 + 1/10×15 + ... + 1/100×105
A = 1/5×5 × (1/1×2 + 1/2×3 + ... + 1/20×21)
A = 1/25 × (1 - 1/2 + 1/2 - 1/3 + ... + 1/20 - 1/21)
A = 1/25 × (1 - 1/21)
A = 1/25 × 20/21
A = 4/105
A = \(\frac{1}{5.10}+\frac{1}{10.15}+...+\frac{1}{100.105}\)
A = \(\frac{1}{5}-\frac{1}{10}+\frac{1}{10}-\frac{1}{15}+...\frac{1}{100}-\frac{1}{105}\)
A = \(\frac{1}{5}-\frac{1}{105}\)
A = \(\frac{20}{105}\)
K MK NHA
\(A=\frac{1}{5.10}+\frac{1}{10.15}+....+\frac{1}{100.105}=\frac{1}{5}\left(\frac{1}{5}-\frac{1}{10}+\frac{1}{10}-\frac{1}{15}+....+\frac{1}{100}-\frac{1}{105}\right)\)
\(\Rightarrow A=\frac{1}{5}\left(\frac{1}{5}-\frac{1}{105}\right)=\frac{1}{5}.\frac{4}{21}=\frac{4}{105}\)
(Dấu chấm . là dấu nhân nha bạn)
\(A=\frac{1}{5\times10}+\frac{1}{10\times15}+.....+\frac{1}{100\times105}\)
\(=\frac{1}{5}-\frac{1}{10}+\frac{1}{10}-......+\frac{1}{100}-\frac{1}{105}\)
\(=\frac{1}{5}-\frac{1}{105}\)
\(=\frac{20}{105}\)
\(A=\frac{1}{5.10}+\frac{1}{10.15}+....+\frac{1}{100.105}\)
\(\Rightarrow5A=\frac{5}{5.10}+\frac{5}{10.15}+....+\frac{5}{100.105}\)
\(\Rightarrow5A=\frac{1}{5}-\frac{1}{10}+\frac{1}{10}-\frac{1}{15}+.....+\frac{1}{100}-\frac{1}{105}\)
\(\Rightarrow5A=\frac{1}{5}-\frac{1}{105}\)
\(\Rightarrow5A=\frac{4}{21}\)
\(\Rightarrow A=\frac{4}{21}:5\)
\(\Rightarrow A=\frac{4}{105}\)
