algsys([a*b=c*d,[a,b,c,d])
returns
%R16 %R17
[[a = ---------, b = %R15, C = %R16, D = %R17]]
%R15
That misses the solutions with b=0, which I suppose could be covered either
by
[[a=%r1,b=%r2*%r3/%r1,c=%r2,d=%r3]]
or
[[a=%r1,b=0,c=0,d=%r2],
[a=%r1,b=0,c=%r2,d=0]]
-s
(MACRAK@MIT-MC 1971-77)