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
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