矩阵的初等变换,若方程组{x1+ax2+x3+x4=1;2x1+x2+bx3+x4=4;2x1+2x2+3x3+cx4=1}与{x1+x2+x3+x4=1;-x2+2x3-x4=2;x3+2x4=-1}同解,试确定a,b,c