P3 [152 points ] (a) Let APPROX-SUM ={S,t,uS={x1,x2,,xk} and for some {y1,y2,,yl}{x1,x2,,xk}, we have tyiu} Prove that APPROX-SUM is NP-complete. (15p) (b) Let SPARSE-SUBGRAPH ={G,kG is a graph that contains a subgraph with k vertices and at most k edges. } Prove that SPARSE-SUBGRAPH is NP-complete. (15p)