Đặt S= 1.2 + 2.3 + 3.4 + ...+ 99.100
3S = 1.2.3+2.3.3+3.4.3+...+98.99.3+99.100.3
3S= 1.2.3+2.3(4-1)+3.4(5-2)+...+98.99(100-97)+99.100(101-98)
3S= 1.2.3+2.3.4-1.2.3+3.4.5-2.3.4+...-97.98.99+99.100.101-98.99.100
3S = 99.100.101 3S = 3.33.100.101
S=33.100.101= 333300
1.2.3.4.5..............97.98.99
C=3.4.5+5.6.7+7.8.9+.....+97.98.99
Tính:
1.2.3+2.3.4+3.4.5+4.5.6+...+97.98.99+98.99.100
Tính tổng:
A=1.2.3+2.3.4+3.4.5+...+97.98.99
Tính nhanh: A=1.2.3+2.3.4+3.4.5+...+97.98.99+98.99.100
1.2.3.4.....2020.2021-1.2.3.4...2019.2020-1.2.3.4...2019.2020^2
1.2.3.4.........2017 -1.2.3.4.......2016-1.2.3.4......2016^2
Tính Q
Q= 1/1.2.3 + 1/3.4.5 ....+1/ 97.98.99
A=1.2.3.4..100-1.2.3.4...99-1.2.3.4...98.992
