2] (15 pts.) The efficiency of an algorithm is given by this sum: C(N)=15+20+25+30++5N, where N is the size of the problem. (N must be 3, do you see why?) a. Using the appropriate summation formula from Appendix A, find a closed-form exact expression (no approximations!) for C(N) : b. Use the closed-form expression found above to approximate the function C(N) by "throwing away" all the low-order terms (but do keep the constant coefficient of the highest-order term!) c. Use the approximation found above to calculate the order of growth . Use the simplest function of N inside theta