If f(x)=ax+bcx+df(x) = \frac{ax+b}{cx+d}f(x)=cx+dax+b and f(f(x))=xf(f(x)) = xf(f(x))=x for x≠−d/cx \neq -d/cx=−d/c, what is the necessary condition for the constants?
d=−ad = -ad=−a
d=ad = ad=a
b=cb = cb=c
ad=bcad = bcad=bc