Question

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

Answer 1

4b=1.2.3.4 +2.3.4.4+...........+(n-1)n(n+1).4

4b=1.2.3.4 +2.3.4.(5-1)+...........+(n-1)n(n+1)(n+2-n-2)

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

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

b=(n-1)n(n+1)(n+2) chia 4 bạn nhé