a.) Let {an} be the sequence given explicitly by an = 1 2(n2 ? n). Find explicitly a1, a2, a5 and a10. b.) Show that an+1 = 1 2(n2 + n), and find a formula for an+1? an. c.) Conclude that an+1 = an + n, with a1 = 0 gives a recursive formula for an+1 in terms of an.