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3x（1x2+2x3+3x4+...99x100)
=3x(1/3(1x2x3-0x1x2+2x3x4-1x2x3+3x4x5-2x3x4+???+98x99x100-97x98x99+99x100x101-98x99x100))
=99x100x101