Question

Tính : C = 1/10 + 1/15 + 1/21 + ... + 1/ 120

Answer 1

\(C=\frac{2}{20}+\frac{2}{30}+\frac{2}{42}+...+\frac{2}{240}=2\times\left(\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+...+\frac{1}{240}\right)\)

\(C=2\times\left(\frac{1}{4\times5}+\frac{1}{5\times6}+\frac{1}{6\times7}+...+\frac{1}{15\times16}\right)\)

\(C=2\times\left(\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+...+\frac{1}{15}-\frac{1}{16}\right)=2\times\left(\frac{1}{4}-\frac{1}{16}\right)=\frac{3}{8}\)

Answer 2

c = 2/20 + 2/30 +2/42 + ... 2 /240

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

=2. (1/4.5 + 1/5.6 + 1/6.7 +...+1/15.16)

= 2.(1/4-1/5+1/5-...-1/16)

=2.(1/4-1/6)=2.3/16=3/8.

ghi nho dau cham la dau nhan