等差数列{an}满足a4等于7,a7等于1,则an等于
问题描述：
等差数列{an}满足a4等于7,a7等于1,则an等于
答
a4=a1+3d=7
a7=a1+6d=1
a1=13 d=-2
an=a1+(n-1)d=13-2(n-1)=-2n+15