If f(x)= (ax+b)/(cx+d), abcd ≠ 0 and f(f(x))=x for all x in the domain of f, what is the value of a+d?
f(f(x)) = (a(ax + b)/(cx + d) + b)/(c(ax + b)/(cx + d) + d).
Expanding and matching coefficients gives you a + d = 2.
+69 
