\(D=\left(1+\frac{1}{2}\right).\left(1+\frac{1}{3}\right).\left(1+\frac{1}{4}\right)...+\left(1+\frac{1}{2018}\right)\)
\(=\frac{3}{2}.\frac{4}{3}......\frac{2018}{2017}.\frac{2019}{2018}\)
\(=\frac{3.4.5....2018.2019}{2.3.4.5....2017.2018}=\frac{2019}{2}\)
\(E=\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+...+\frac{1}{79.80}+\frac{1}{80.81}\)
\(=\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+....+\frac{1}{80}-\frac{1}{81}\)
\(=\frac{1}{4}-\frac{1}{81}\)
\(=\frac{77}{324}\)
\(\text{D}=\left(1+\frac{1}{2}\right)\cdot\left(1+\frac{1}{3}\right)\cdot...\cdot\left(1+\frac{1}{2017}\right)\cdot\left(1+\frac{1}{2018}\right)\)
\(\text{D}=\frac{3}{2}\cdot\frac{4}{3}\cdot\frac{5}{4}\cdot...\cdot\frac{2018}{2017}\cdot\frac{2019}{2018}\)
\(\text{D}=\frac{2019}{2}\)
\(\text{E}=\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+...+\frac{1}{79.80}+\frac{1}{80.81}\)
\(\text{E}=\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+...+\frac{1}{79}-\frac{1}{80}+\frac{1}{80}-\frac{1}{81}\)
\(\text{E}=\frac{1}{4}-\frac{1}{81}=\frac{81}{324}-\frac{4}{324}=\frac{77}{324}\)
E = 1/4 - 1/5 + 1/5 - 1/6 + 1/6 - 1/7 +...+ 1/79 - 1/8 + 1/80 - 1/81
= 1/4 - 1/81
= 77/34
mk chỉ bt lm vậy thôi!
D\(=\left(1+\frac{1}{2}\right).\left(1+\frac{1}{3}\right).\left(1+\frac{1}{4}\right)...\left(1+\frac{1}{2017}\right).\left(1+\frac{1}{2018}\right)\)
\(D=\frac{3}{2}.\frac{4}{3}.\frac{5}{4}...\frac{2018}{2017}.\frac{2019}{2018}\)
\(D=\frac{3.4.5...2018.2019}{2.3.4...2017.2018}\)
\(D=\frac{2019}{2}\)
\(E=\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+...+\frac{1}{79.80}+\frac{1}{80.81}\)
\(E=\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+...+\frac{1}{79}-\frac{1}{80}+\frac{1}{80}-\frac{1}{81}\)
\(E=\frac{1}{4}-\frac{1}{81}\)
\(E=\frac{77}{324}\)
D = ( 1 + 1/2 ) . ( 1 + 1.3 ) . ( 1 + 1/4 ) ...... ( 1 + 1/2017 ) . ( 1 + 1/ 2018 )
= 3/2 . 4/3 . 5/4 ...... 2018/2107 . 2019/2018
= 3.4.5.......2018.2019/2.3.4.....2017.2018
= 2019/2
E = 1/4.5 + 1/5.6 + 1/6.7 + ..... + 1/79.80 + 1/80.81
= 1/4 - 1/5 + 1/5 - 1/6 + ..... + 1/79 - 1/80 + 1/80 - 1/81
= 1/4 - 1/81
= 77/324
