Question

1*2+2*3+.....+9*10

Answer 1

330

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Answer 2

A=1.2+2.3+..+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

A=(9.10.11):3=330

Answer 3

Đặt A=1*2+2*3+.......+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

A=9*10*11:3

A=330