5. Suppose $f(t,u)$ is a biquadratic polynomial function that is 0 at all the points $(i,j)$ for $i = 0,1,2$ and $j = 0,1,2$, except that $f(1,1) = 1$. Find $f(\frac 12, \frac 12)$. (There is only one function that fits this description, so if you can think of one, that's it.)