求一个矩阵的值 但是矩阵里面含有x1，x2这些未知量，需要把x1，x2等当作已知的数值进行行列式运算
> c
     [,1] [,2] [,3] [,4]
[1,] "x1" "x2" "0"  "x7"
[2,] "0"  "1"  "x3" "x8"
[3,] "0"  "x3" "1"  "x9"
[4,] "x5" "0"  "x4" "x10"
>

这是运算规则：
0*x1=x1
1*x1=x1
x1*x2=x1x2