Let A-{x1,x2 , a S 10, 1 Si s 10. Prove that there are at least two different 5-element subsets S1 and S2 of A such that the sum of the elements in Si is equal to the sum of the elements in S2 (20 points) Hint: Use the pigeonhole principle. 4. , xo) be a set of 10 positive integers, not necessarily distinct, such that