n=2
a = [ [2 1.0]' [-200 0.01]']
b= [200 1]'
r=a^(-1) * b

x=[0 0];
m21=a(2,1)/a(1,1);
a(2,1)=0;
a(2,2) = radd(a(2,2),-m21*a(1,2),n);
b(2)= radd(b(2),-m21*b(1),n);

x(2)=b(2)/a(2,2);
x(1) = (radd(b(1), -a(1,2)*x(2),n))/a(1,1);
x'