计算①1x2/1+2x3/1+3x4/1+```+19x20/1= ②1x4/1+4x7/1+7x10/1+```+91x94/1=

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计算①1x2/1+2x3/1+3x4/1+```+19x20/1= ②1x4/1+4x7/1+7x10/1+```+91x94/1=
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计算①1x2/1+2x3/1+3x4/1+```+19x20/1= ②1x4/1+4x7/1+7x10/1+```+91x94/1=
计算①1x2/1+2x3/1+3x4/1+```+19x20/1= ②1x4/1+4x7/1+7x10/1+```+91x94/1=

计算①1x2/1+2x3/1+3x4/1+```+19x20/1= ②1x4/1+4x7/1+7x10/1+```+91x94/1=
是否写反了
差/(小*大) =1/小 - 1/大 如 7/(3*10)=1/3 - 1/10 ; 1/(4*5)=1/4 - 1/5
但 1/(4*7) =3/(4*7) *(1/3)=(1/3)[1/4 -1/7]
1/(1*2)+1/(2*3)+1/(3*4)+...+1/(19*20)=1/1 -1/2+ 1/2 - 1/3 +1/3 - 1/4 +...+1/18- 1/19+1/19 -1/20
=1/1 -1/20 = 19/20...ANS
速算 大分子=各项分子和=1*19=19 ;大分母=第一项分母头*最后项分母尾=1*20=20 =>19/20
1,4,7,...,91等差 项数=(91-1)/3 +1=31
1/(1*4)+1/(4*7)+1/(7*10)+.+1/(91*94) =[1*31]/(1*94)=31/94...ANS
OR =1/3[1/1 - 1/4 +1/4 -1/7 +...+1/88- 1/91+1/91-1/94]=(1/3)(1- 1/94) =31/94

用克拉默法则解下列方程组 x1-2x2+3x3-4x4=4 x2-x3+x4=-3 x1+3x2+2x4=1 -7x2+3x3+x4=3x1-2x2+3x3-4x4=4x2-x3+x4=-3 x1+3x2+2x4=1 -7x2+3x3+x4=3 因式分解：x4+x3+2x2 +x+1 解方程组X1-2x2+3x3-x4=1,3x1-x2+5x3-3x4=2,2x1+x2+2x3-2x4=3 x1+5x2-x3-x4=-1x1-2x2+x3+3x4=33x1+8x2-x3+x4=1 写出方程组2*x1+x2-x3+x4=1,x1+2*x2+x3-x4=2,x1+x2+2*x3+x4=3的通解? 求非齐次线方程组的通解 ：2x1+x2-x3+x4=1 x1+2x2+x3-x4=2 x1+x2+2x3+x4=3 具体写出方程组：2x1+x2-x3+x4=1;x1+2x2+x3-x4=2;x1+x2+2x3+x4=3的通解 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=2 100x100-99x99+98x98-97x97+.+4x4-x4-3x3+2x2-1计算 计算:1/1x2+1/2x3+1/3x4+.+1/(n-1)n 计算：1/1x2+1/2x3+1/3X4+...+1/9x10=? 计算:1/1x2+1/2X3+1/3x4+...+1/2009x2010 计算 1/1x2+1/2x3+1/3x4+.+1/2009x2010 计算：1/1x2+1/2x3+1/3x4+.+1/2010X2011 计算：1/1x2+1/2x3+1/3X4+...+1/9x10=? 1/1x2+1/2x3+1/3x4+...+1/49x50怎么计算 计算1x2+2x3+3x4+4x5+...+99x100 计算1x2+2x3+3x4+……+99x100=?