Đặt A= 1.2.3+2.3.4+3.4.5+...+101.102.103
=>4A=1.2.3.4+2.3.4.4+3.4.5.4+...+101.102.103.4
=1.2.3.(4-0)+2.3.4.(5-1)+3.4.5.(6-2)+...+101.102.103.(104-100)
=1.2.3.4-0.1.2.3+2.3.4.5-1.2.3.4+3.4.5.6-2.3.4.5+...+101.102.103.104-100.101.102.103
=101.102.103.104-0.1.2.3
=110355024
=>A=110355024:4=27588756
P = 1/1.2.3 + 1/2.3.4 + 1/3.4.5 +...+ 1/n(n+1)(n+2)
S = 1/1.2.3 + 1/2.3.4 + 1/3.4.5 +...+ 1/48.49.50 .
1.2.3+2.3.4+3.4.5+...+98.99.100
Tinh nhanh
A= \(1.2.3+2.3.4+3.4.5+...+48.49.50\)
B = \(1.2.3+2.3.4+3.4.5+...+n.\left(n+1\right).\left(n+2\right)\)
1.2.3 + 2.3.4 + 3.4.5
B=1.3+3.5+5.7+...+97.98
C=1.2.3+2.3.4+3.4.5+4.5.6+5.6.7+7.8.9+8.9.10
D=1.2.3+2.3.4+...+99.100.101
Tính : a, 1.2.3 + 2.3.4 + 3.4.5 + ... + (n - 1).n.(n+1)
b, 1.2.3 + 3.4.5 + 5.6.7 + 98.99.100
1.2.3+2.3.4+3.4.5+..............+2014.2015.2016 = ?
1.2.3+2.3.4+3.4.5+98.99.100
1.2.3+2.3.4+3.4.5....98.99.100
