<=>4S=1.2.3.4 + 2.3.4.4+3.4.5.4+.....+8.9.10.4
<=>4S =1.2.3.4 + 2.3.4.(5-1) + 3.4.5.(6-2)+......+8.9.10.(11-7)
<=>4S=1.2.3.4 + 2.3.4.5 -1.2.3.4+3.4.5.6- 2.3.4.5+......+8.9.10.11 - 7.8.9.10
<=> 4S=8.9.10.11
<=>S=1980
Tính: 1/1.2.3+1/2.3.4+1/3.4.5+.........+1/8.9.10
- Tính giá trị biểu thức : C=1.2.3+2.3.4+3.4.5+4.5.6+5.6.7+6.7.8+7.8.9+8.9.10
D= 1.2.3+2.3.4+....+8.9.10
1.2.3+2.3.4+3.4.5+...+8.9.10
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
B=1/1.2.3+1/2.3.4+...+1/8.9.10
1/1.2.3+1/2.3.4+...+1/8.9.10
1/1.2.3+1/2.3.4+...+1/8.9.10 =?
1.2.3+2.3.4+3.4.5+4.5.6+5.6.7+6.7.8+7.8.9+8.9.10
