Question

Tính B= 1.2.3+2.3.4+3.4.5+...+(n-1)n(n+1)

Answer 1

4(1.2.3) = 1.2.3.4 - 0.1.2.3

4(2.3.4) = 2.3.4.5 - 1.2.3.4

4(3.4.5) = 3.4.5.6 - 2.3.4.5

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4(n-1)n(n+1) = (n-1)n(n+1)(n+2) - (n-2)(n-1)n(n+1)

=> 4 B = (n-1)n(n+1)(n+2) => B= (n-1)n(n+1)(n+2):4

Answer 2

4(1.2.3)=1.2.3.4 - 0.1.2.3

4(2.3.4)=2.3.4.5 - 1.2.3.4

4(3.4.5)=3.4.5.6 - 2.3.4.5

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Answer 3

thank

Answer 4

B= 1.2.3+2.3.4+3.4.5+...+(n-1)n(n+1)

4B=1.2.3.(4-0) + 2.3.4.(5-1) + ... +(n-1).n.(n+1).[n+2-(n-2)]

4B=1.2.3.4-1.2.3.4+2.3.4.5+...+(n-2).(n-1).n.(n+1) + (n-1).n.(n+1).(n+2)

=> 4B=(n-1).n.(n+1).(n+2)

=> B= (n-1).n.(n+1).(n+2):4