Question

s= 1/10+1/15+1/21+...+1/120

mik cần giải gấp bài này

ai jup mik nha

Answer 1

\(S=\frac{1}{10}+\frac{1}{15}+\frac{1}{21}+...+\frac{1}{120}\)

\(S=\frac{2}{20}+\frac{2}{30}+\frac{2}{42}+...+\frac{2}{240}\)

\(S=2.\left(\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+...+\frac{1}{15.16}\right)\)

\(S=2.\left(\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+...+\frac{1}{15}-\frac{1}{16}\right)\)

\(S=2.\left(\frac{1}{4}-\frac{1}{16}\right)\)

\(S=2.\left(\frac{4}{16}-\frac{1}{16}\right)\)

\(S=2.\frac{3}{16}=\frac{3}{8}\)

Answer 2

      S=1/2.5 + 1/5.3 + 1/3.7+ ...+ 1/15.8

1/2 S=1/4.5 + 1/5.6 + 1/6.7 + ...+ 1/15.16

1/2 S=1/4-1/5+1/5-1/6+1/6-1/7+...+1/15-1/16

1/2 S=1/4-1/16

1/2 S=3/16

     S=3/16:1/2=3/8