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, which condition must be satisfied by the constants a,b,c,da, b, c, da,b,c,d?
a+d=0a + d = 0a+d=0
a=da = da=d
b=cb = cb=c
ad−bc=0ad - bc = 0ad−bc=0