ko ghi lại đề
\(=1+1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+...+\frac{1}{210}-\frac{1}{211}+\frac{1}{211}-\frac{1}{212}\)
\(=1-\frac{1}{212}\)
\(=\frac{211}{212}\)
\(\frac{1}{8.9}+\frac{1}{9.10}+...+\frac{1}{211.212}\)
= \(\frac{1}{8}-\frac{1}{9}+\frac{1}{9}-\frac{1}{10}+...+\frac{1}{211}-\frac{1}{212}\)
= \(\frac{1}{8}-\frac{1}{212}\)
= \(\frac{51}{424}\)
\(\frac{1}{8.9}+\frac{1}{9.10}+\frac{1}{10.11}+...+\frac{1}{210.211}+\frac{1}{211.212}\)
=\(\frac{1}{8}-\frac{1}{9}+\frac{1}{9}-\frac{1}{10}+\frac{1}{10}-\frac{1}{11}+...+\frac{1}{210}-\frac{1}{211}+\frac{1}{211}-\frac{1}{212}\)
=\(\frac{1}{8}-\frac{1}{212}\)=\(\frac{53}{424}-\frac{2}{424}=\frac{51}{424}\)\(\approx0,12028\approx0,12\)
1/8.9 + 1/9.10 + 1/10.11 + ... + 1/210.211 + 1/211.212
= 1/8 - 1/9 + 1/9 - 1/10 + 1/10 - 1/11 + ... + 1/210 - 1/211 + 1/211 - 1/212
= 1/8 - 1/212
= 51/424
