1x2＋2x3＋3x4……＋19x20=?

1x2＋2x3＋3x4……＋19x20=? 1x2+2x3+3x4+……+19x20如何巧算? 1x2+2x3+3x4+.+19x20 1/1x2 + 1/2x3 + 1/3x4 +...+1/19x20(急） 1/1x2+1/2x3+1/3x4+.1/19x20 在线等 急 求解 1/1x2+1/2x3+1/3x4+...+1/19x20 请说出道理 (2X2)/1X3+(4X4)/(3X5)+(6X6)/(5X7)+...+(20X20)/(19X21)==? 1/1x3+1/2x4+1/3x5+……+1/18x20=? 2x3+3x4+4x5+.+19x20=? 1x2/1+2x3/1+3x4/1+4x5/1.19x20/1 用克莱姆法则解线性方程!x1＋x2+2x3＋3x4=13x1-x2-x3-2x4=-42x1+3x2-x3-x4＝-6x1+2x2+3x3-x4=-4 1/2x3十1/3x4十1/4x5十……十1/18x19十1/19x20=? 求非齐次线性方程组X1-X2-X3+X4=0,X1－X2＋X3－3X4=1,2X1－2X2－4X3＋6X4=－1的通解 观察下列单项式：﹣x,2x2,﹣3x3,4x4,…,﹣19x19,20x20…,则第2013个单项式为（ ）,第n个单项式观察下列单项式：﹣x,2x2,﹣3x3,4x4,…,﹣19x19,20x20…,则第2013个单项式为（ ）,第n个单项式为（ ）,第n+1个 计算①1x2/1+2x3/1+3x4/1+```+19x20/1= ②1x4/1+4x7/1+7x10/1+```+91x94/1= 运筹学……对偶问题.maxz=x1+2x2+3x3+4x4-x1+x2-x3-3x4=56x1-7x2+3x3-5x4 1/1X3+1/2X4+1/3X5+.+1/18X20=? 1/1x3+1/2x4+1/3x5.1/18x20=?简便算法,