Đặt biểu thức là A
\(\Rightarrow2A=\frac{2}{1.2.3}+\frac{2}{2.3.4}+\frac{2}{3.4.5}\)
\(\Rightarrow2A=\frac{3-1}{1.2.3}+\frac{4-2}{2.3.4}+\frac{5-3}{3.4.5}=\frac{1}{1.2}-\frac{1}{2.3}+\frac{1}{2.3}-\frac{1}{3.4}+\frac{1}{3.4}-\frac{1}{4.5}\)
\(\Rightarrow2A=\frac{1}{2}-\frac{1}{20}=\frac{9}{20}\Rightarrow A=\frac{9}{40}\)
\(\dfrac{1}{1.2.3}+\dfrac{1}{2.3.4}+.....+\dfrac{1}{37.38.39}\)
\(\dfrac{1}{1.2.3}+\dfrac{1}{2.3.4}+...+\dfrac{1}{48.49.50}\)
tính tổng S=1/1.2.3+1/2.3.4+...+1/78.79.80
Tính 1/1.2.3-1/2.3.4-1/3.4.5-.......-1/97.98.99
Tính 1/1.2.3 -1/2.3.4 -1/3.4.5- .....-1/97.98.99
tinh A=1/1.2.3+1/2.3.4+....+1/99.100.101
Tính: 1/1.2.3+1/2.3.4+1/3.4.5+...+1/99.100.101
Tính nhanh tổng sau: 1/1.2.3+1/2.3.4+1/3.4.5+...+1/10.11.12
1/1.2.3+1/2.3.4+1/3.4.5+...+1/18.19.20
help me please:))))) thanks
