[R,j]=rref(A)
A是矩阵
R是简化后的阶梯形
j是主元

>> A = [1 1 -2 1 4; 2 4 -6 4 8; 2 -3 1 -1 2; 3 6 -9 7 9]; 
>> [R,j]=rref(A)

R =

     1     0    -1     0     4
     0     1    -1     0     3
     0     0     0     1    -3
     0     0     0     0     0


j =

     1     2     4

>>