A = 1/1.2.3 + 1/2.3.4 + ... + 1/18.19.20
\(A=\frac{1}{2}.\left(\frac{1}{1.2}-\frac{1}{2.3}+\frac{1}{2.3}-\frac{1}{3.4}+...+\frac{1}{18.19}-\frac{1}{19.20}\right)\)
\(A=\frac{1}{2}.\left(\frac{1}{1.2}-\frac{1}{19.20}\right)\)
\(A=\frac{1}{4}-\frac{1}{2.19.20}< \frac{1}{4}\)
1/1.2.3+1/2.3.4+1/3.4.5+...+1/18.19.20
help me please:))))) thanks
CMR: 1/(1.2.3)+1/(2.3.4)+...+1/(98.99.100)=4949/19800
Cho N = 1.2.3 + 2.3.4+...+n(n+1)(n+2) . CMR : 4N+1 là số chính phương
Cho A=1/1.2.3+1/2.3.4+1/3.4.5+...+1/2014.2015.2016 so sánh A với 1/4
Tính :B=1.2.3+2.3.4+............+(n-1)n(n+1).4
tinh A=1/1.2.3+1/2.3.4+....+1/99.100.101
Tính nhanh:
5) 1/2 + 1/2^2 + 1/2^3 + 1/2^4 +...+ 1/2^100
6) 1/1.2.3 + 1/2.3.4 + ... + 1/48.49.50
7) -1/3 + 1/3^2 - 1/3^3 + ... + 1/3^100 - 1/3^101
8) 1/1.2.3 + 1/2.3.4 + ... + 1/37.38.39
C=1/1.2.3+1/2.3.4+1/3.4.5+...+1/a.(a+1).(a+2)
Tính A = 1.2.3 + 2.3.4 + ... + (n - 1)n(n + 1)
