1x2+2x3+3x4+4x5+5x6+6x7+7x8+.+nx(n+1) 要公式,别搞太深奥!

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1x2+2x3+3x4+4x5+5x6+6x7+7x8+.+nx(n+1)要公式,别搞太深奥!1x2+2x3+3x4+4x5+5x6+6x7+7x8+.+nx(n+1)要公式,别搞太深奥!1x2+2x

1x2+2x3+3x4+4x5+5x6+6x7+7x8+.+nx(n+1) 要公式,别搞太深奥!
1x2+2x3+3x4+4x5+5x6+6x7+7x8+.+nx(n+1) 要公式,别搞太深奥!

1x2+2x3+3x4+4x5+5x6+6x7+7x8+.+nx(n+1) 要公式,别搞太深奥!
1x2+2x3+3x4+…+n(n+1)
=1x(1+1)+2x(2+1)+3x(3+1)+…n(n+1)
=(1^2+2^2+3^2+…+n^2)+(1+2+3+…+n)
=n(n+1)(2n+1)/6+n(n+1)/2
=n(n+1)[(2n+1)+3]/6