mình nghĩ là 1/5 chứ không phải 1/6
1/3 chứ ko phải 1/6
ai đồng ý thì k cho mk nhé
\(A=\frac{1.2}{6.2}+\frac{1.2}{10.2}+....+\frac{1.2}{55.2}\)
\(A=\frac{2}{12}+\frac{2}{20}+...+\frac{2}{110}\)
\(A=\frac{2}{3.4}+\frac{2}{4.5}+...+\frac{2}{10.11}\)
\(A=2\left(\frac{1}{3.4}+\frac{1}{4.5}+...+\frac{1}{10.11}\right)\)
\(A=2\left(\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+....+\frac{1}{10}-\frac{1}{11}\right)\)
\(A=2\left(\frac{1}{3}-\frac{1}{11}\right)\)
\(A=2.\frac{8}{33}\)
\(A=\frac{16}{33}\)
\(A=\frac{1}{6}+\frac{1}{10}+\frac{1}{15}+...+\frac{1}{55}=\frac{2}{12}+\frac{2}{20}+\frac{2}{30}+...+\frac{2}{110}=2.\left(\frac{1}{3x4}+\frac{1}{4x5}+\frac{1}{5x6}+...+\frac{1}{10x11}\right)\)
\(=2.\left(\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+...+\frac{1}{10}-\frac{1}{11}\right)=2.\left(\frac{1}{3}-\frac{1}{11}\right)=2.\left(\frac{11}{33}-\frac{3}{33}\right)=2.\frac{8}{33}=\frac{16}{33}\)
