3. Again, most people got this. Some of you made small mistakes with the signs. A couple solutions left things in the form $r \leq | a_i c - b_i|$, and either did not attempt to say why this was OK in an LP (it is not!), or claimed the techniques of problem 2 handled this case (they don’t, since the solution space is not convex in this case!) — you’ve got to be careful with absolute values in LPs.
4. Here a common mistake was to bound the maximum numerical value of the determinant and claim that $size(det(A))$ was at most the log of that quantity. But note that the size of the determinant could be large also because its value is a rational number with a huge denominator (and hence requires a lot of bits to write down, even though $det(A)$ potentially has a small numerical value).