Question

tinh nhanh 

N = 1/6 + 1/12 + 1/20 + ......+ 1/90

Answer 1

\(N=\frac{1}{2.3}+\frac{1}{3.4}+.......+\frac{1}{9.10}\)

\(N=\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+......+\frac{1}{9}-\frac{1}{10}\)

\(N=\frac{1}{2}-\frac{1}{10}\)

\(N=\frac{2}{5}\)

Answer 2

\(N=\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+...+\frac{1}{90}\)

\(N=\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+...+\frac{1}{9.10}\)

\(N=\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+...+\frac{1}{9}-\frac{1}{10}\)

\(N=\frac{1}{2}-\frac{1}{10}\)

\(N=\frac{2}{5}\)