19. Given the following terms of a sequence, find and state both the nth term definition and the recursive definition. Show all work, make sure your definitions are complete, and state your answer in terms of n (not n-1) for the nth term. 3, 7, 13, 21, 31, 43, ... 20. Find (a) the interval of convergence, and (b) the sum, expressed in terms of x, for the following series. 1 X + 21. Express the given series using summation notation. UIN N/ - + DO / W 11