If \(f(x)=\frac{ax+b}{cx+d}, abcd\not=0\) and \(f(f(x))=x\) for all \(x\)in the domain of \(f\), what is the value of \(a+d\)?
This was asked before but the thread got locked and no one provided a soluton
If you try some cases, you'll find a + d = 1.
that is the wrong answer, but it's ok! the answer is actually 0
+680 
