pivotagem parcial
function [a,b]=pivparcial(a,b,n)
for k=1:(n-1)
    max=abs(a(k,k));
    l=k;
    for i=k+1:n
        if abs(a(i,k))>max
            max=abs(a(i,k));
            l=i;
        end
    end
    if l~=k
        for j=k:n
            aux=a(l,j);
            a(l,j)=a(k,j); 
            a(k,j)=aux;
        end
        aux=b(l);
        b(l)=b(k); 
        b(k)=aux;
    end
    for i=k+1:n
        mult=a(i,k)/a(k,k);
        a(i,k)=0;
        for j=k+1:n
            a(i,j)=a(i,j)-mult*b(k);
        end
        b(i)=b(i)-mult*b(k);
    end
end
a(i,j)
b(i)
end