3S = 1.2.3 + 2.3.3+......+99.100.3
3S = 1.2.3 + 2.3.(4-1) +.......+99.100.(101-98)
3S = 1.2.3 +2.3.4-1.2.3+......+99.100.101 - 99.100.101
3S = (1.2.3-1.2.3) + (2.3.4-2.3.4) +...... + 99.100.101
3S = 99.100.101 = 999900
=> S = 333300
Vậy S = 333300
a) Chứng tỏ:(a+1).(a+2).(a+3)-a.(a+1).(a+2)=3.(a+1).(a+2)
b) Áp dụng phần trên để tính:S=1.2+2.3+3.4+4.5+5.6+........+99.100
A=1.2+2.3+3.4+...+99.100
1.2+2.3+3.4+4.5+...+99.100
1.2+2.3+3.4+.....99.100 =?
1.2+2.3+3.4+...+99.100
Tính:
B = 1.2 + 2.3 + 3.4 + ... + 99.100
1.2+2.3+3.4+....+99.100=???
1.2+2.3+3.4+...+99.100=?
1.2+2.3+3.4+...+99.100
1.2+2.3+3.4+...+99.100
