26.已知数列{an}\{a_{n}\}{an}满足an=n2−5n−6a_{n}=n^{2}-5n-6an=n2−5n−6,n∈N+n\in N_{+}n∈N+(Ⅰ)数列中有哪些项是负数?(Ⅱ)当nnn为何值时,ana_{n}an取得最小值?并求出此最小值.