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CMR: 1n+2n+3n+....+xn" role="presentation" style="font-size: 13.696px; display: inline; word-spacing: 0px; position: relative;">1n+2n+3n+....+xn1n+2n+3n+....+xn chia hết cho 1+2+3+...+x" role="presentation" style="font-size: 13.696px; display: inline; word-spacing: 0px; position: relative;">1+2+3+...+x1+2+3+...+x với x;n" role="presentation" style="font-size: 13.696px; display: inline; word-spacing: 0px; position: relative;">x;nx;n là các số nguyên dươ