Question

S=6/15*18+6/18*21+6/21*24+...+6/87*90

giup toi voi

 

Answer 1

\(s=\frac{6}{15\times18}+\frac{6}{18\times21}+\frac{6}{21\times24}+....+\frac{6}{87\times90}\)

\(s-3=\frac{3}{15\times18}+\frac{3}{18\times21}+\frac{3}{21\times24}+....+\frac{3}{87\times90}\)

\(s-3=\frac{1}{15}-\frac{1}{18}+\frac{1}{18}-\frac{1}{21}+\frac{1}{21}-\frac{1}{24}+....+\frac{1}{87}-\frac{1}{90}\)

\(s-3=\frac{1}{15}-\frac{1}{90}\)

\(s-3=\frac{1}{18}\)

\(s=\frac{1}{18}+3\)

\(s=\frac{55}{18}\)