Question

tính nhanh : A = 1/10+1/15+1/21+........+1/66

Answer 1

\(A=\frac{1}{10}+\frac{1}{15}+\frac{1}{21}+...+\frac{1}{66}\)

\(A=\frac{2}{20}+\frac{2}{30}+\frac{2}{42}+...+\frac{2}{132}\)

\(A=\frac{1}{2}.\left(\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+...+\frac{1}{132}\right)\)

\(A=\frac{1}{2}.\left(\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+...+\frac{1}{11.12}\right)\)

\(A=\frac{1}{2}.\left(\frac{1}{4}-\frac{1}{12}\right)\)

\(A=\frac{1}{2}.\frac{1}{6}\)

\(A=\frac{1}{12}\)