X is a set of integers containing greater than or equal to 8.f:X-X,f(x+y)=f(xy),x>=4,y>=4,f(8)=9,f(9)=?

Answer 1

Your question is incomplete

Answer 2

Your question is incomplete.However, assuming f to be bijective,
we have
x+y=xy
or y=x/(x-1). Notice that this ratio is of greater inequality,i.e,.. if you add the same value to both numerator and denominator its value decreases.
for x=4, y=4/3
forx=infinity, y=1(take the limit)
So, y should lie between 4/3 and 1.
But for any such value of y, x+y<8.
Thus, x+y does not belong to X and there are no solutions.