Ta chia E cho 2
=> E/2=1.2+2.3+3.4+4.5+...+98.99+99.100
=> áp dụng công thức ta tính dc E= 666600
E=4+12+24+40+...+19404+19800
1/2E=2+6+12+...+9702+9900
1/2E=1.2+2.3+3.4+...+98.99+99.100
3/2E=1.2,3+2.3.(4-1)+3.4.(5-2)+...+98.99.(100-97)+99.100.(101-98)
3/2E=1.2.3+2.3.4-1.2.3+3.4.5-2.3.4+...+98.99.100-97.98.99+99.100.101-98.99.100
3/2E=99.100.101
E=(99.100.101):3/2=666600
F=4+12+24+40+...+19404+19800
F:2=1.2+2.3+3.4+4.5+...+98.99+99.100
3.(F:2)=1.2.3+2.3.3+3.4.3+4.5.3+...+98.99.3+99.100.3
3.(F:2)=1.2.3+2.3.(4-1)+3.4.(5-2)+4.5.(6-3)+...+98.99.(100-97)+99.100.(101-98)
minh chi lam den day thui nhe
1/2 E= 1.2+.2.3+3.4+4.5+...+98.99+99.100
E= 666600
