Question

tính tổng A hợp lí biết  A=10/11x16+10/16x21+10/21x26+...+10/61x66

Answer 1

\(A=\frac{10}{11.16}+\frac{10}{16.21}+...+\frac{10}{61.66}\)

\(A=\frac{10}{5}\left(\frac{5}{11.16}+\frac{5}{16.21}+...+\frac{5}{61.66}\right)\)

\(A=\frac{10}{5}\left(\frac{1}{11}-\frac{1}{16}+\frac{1}{16}-\frac{1}{21}+...+\frac{1}{61}-\frac{1}{66}\right)\)

\(A=\frac{10}{5}\left(\frac{1}{11}-\frac{1}{66}\right)=\frac{10}{5}.\frac{5}{66}=\frac{5}{33}\)

Vậy A =5/33