计算：1/1X3+1/2X4+1/3X5+.+1/18X20=?并说明理由 快加把劲

(2X2)/1X3+(4X4)/(3X5)+(6X6)/(5X7)+...+(20X20)/(19X21)==? 解方程组X2+X3+X4=1 X1+X2+X3=5 X3+X4+X5=-5 X4+X5+X1=-3 X5+X1+X2=2 求齐次线性方程组x1+2x2+x3+x4+x5=1 2x1+4x2+3x3+x4+x5=2 -x1-2x2+x3+3x4 求解一个方程急 X（1次方）+X2+X3=5 X2+X3+X4=1 X3+X4+X5=-5 X4+X5+X1=-3 X5 解一道方程组x1+x2+x3=5,x2+x3+x4=1,x3+x4+x5=-5,x4+x5+x1=-3,x5+x1+x2 计算①1x2/1+2x3/1+3x4/1+```+19x20/1= ②1x4/1+4x7/1+7x10/1+```+91 一道简便计算数学题1X2+2X3+3X4+4X5...+n(n+1)一1X2+2X3+3X4+4X5...+2013X2 简便计算1/2+1/2x3+1/3x4+1/4x5+...+1/99x100 计算1/1X3+1/2X4+1/3X5+.+1/10X12 计算1x2+2x3+3x4+4x5+...+99x100 X5-X4+X3+X2+X+1因式分解 x5-x4+x3-x2+x-1因式分解