Đặt A = 1.2 + 2.3 + 3.4 + ... + 9.10
=> 3A = 1.2.3 + 2.3.(4-1) + ... + 9.10.(11-8)
=> 3A = 1.2.3 + 2.3.4 - 1.2.3 + ... + 9.10.11 - 8.9.10
=> 3A = 9.10.11
=> 3A = 990
=> A = 330
Vậy, A = 330
Trả lời :
1.2+2.3+3.4+4.5+5.6+6.7+7.8+8.9+9.10 = 330
Hok tốt
đặt A = 1.2 + 2.3 + 3.4 + ... + 9.10
3A = 1.2.3 + 2.3.3 + 3.4.3 + ... + 9.10.3
3A = 1.2.3 + 2.3.(4 - 1) + 3.4.(5 - 2) + ... + 9.10.(11 - 8)
3A = 1.2.3 + 2.3.4 - 1.2.3 + 3.4.5 - 2.3.4 + ... + 9.10.11 - 8.9.10
3A = 9.10.11
S = 9.10.11 : 3
A = 330
1.2+2.3+3.4+4.5+5.6+6.7+7.8+8.9+9.10
=(9+1).(2+8).(2+8).(3+7).(3+7).(6+4).(6+4).(5+5).(10+9)
=10.10.10.10.10.10.10.10.19
=(10.10).(10.10).(10.10).(10.10)+19
= 100 . 100 . 100 .100 +19
=10000.10000+19
=100000019
=
\(A=1.2+2.3+...+9.10\)
\(3A=1.2.3+2.3.\left(4-1\right)+...+9.10.\left(11-8\right)\)
\(=1.2.3+2.3.4-1.2.3+...+9.10.11-8.9.10\)
\(=9.10.11\)
Suy ra \(A=\frac{9.10.11}{3}=330\)
