So why are you making up utterly pointless homework problems? Just get a job in academia already if that's what you want to do.
Also, your problem is unsolvable as stated.
tD(a,a,a)==tD(a,a,b)==tD(a,b,a)==tD(b,a,a)==FALSE
However, fE(a,a,a,a) != fE(a,a,a,b), but as fE can only be defined in terms of the above expressions, which all have the same value, fE(a,a,a,a) will be equal to fE(a,a,a,b) no matter how it is defined.