Đặt A = 5.6 + 6.7 + 7.8 + 8.9 +...+ 97.98
=> 3A = 5.6.3 + 6.7.3 + 7.8.3 + 8.9.3 + ...+ 97.98.3
3A = 5.6.(7-4) + 6.7.(8-5) + 7.8(9-6) + 8.9.(10-7) +...+ 97.98.(99-96)
3A = 5.6.7 - 4.5.6 + 6.7.8 -5.6.7 + 7.8.9 - 6.7.8 + 8.9.10 - 7.8.9 + ...+ 97.98.99 - 96.97.98
3A = 97.98.99 - 4.5.6
A = 313 658
Tính nhanh :
5.6 + 6.7 + 7.8 + 8.9 + ... + 97.98
Tính nhanh :
5.6 + 6.7 + 7.8 + 8.9 +...+ 97.98
A = 1.2+2.3+3.4+4.5+5.6+6.7+7.8+8.9+7.10
⇒ tự luận
1.1+1.2+2.3+3.4+4.5+5.6+6.7+7.8+8.9+9.10+10
A=1.2+2.3+3.4+4.5+5.6+6.7+7.8+8.9+9.10+...+2016.2017
Tinh 1/3.4-1/4.5-1/5.6-1/6.7-1/7.8-1/8.9
1.2+2.3+3.4+4.5+5.6+6.7+7.8+8.9+9.10
tính 1/5.6 - 1/6.7 - 1/7.8 - 1/8.9 -...- 1/18.19 - 1/19.20
A=1.2+2.3+3.4+4.5+5.6+6.7+7.8+8.9 cach lam
