> restart:
fse:=x->factor(simplify(expand(x))):
f:=Sum((a[1]*x[i]+a[0]-y[i])^2,i=1..n);

> f:=sum((a[1]*x[i]+a[0]-y[i])^2,i=1..n):
sol:=solve({diff(f,a[1])=0,diff(f,a[0])=0},{a[1],a[0]}):
a[0]:=fse(subs(sol,a[0]));
a[1]:=fse(subs(sol,a[1]));

> fse(a[0] - (sum(y[i],i=1..n)-a[1]*sum(x[i],i=1..n))/n);

> restart;
a[0] := (sum(y[i],i=1..n)-a[1]*sum(x[i],i=1..n))/n;

>