己知{an}为等差数列,且a7-2a4=-1,a3=0,则公差d=______.2、已知Sn=1/3(an-1),

题目

己知{an}为等差数列,且a7-2a4=-1,a3=0,则公差d=______.2、已知Sn=1/3(an-1),

答案

a7 - 2a4 = a7 - (a3 + a5) = a7 - a5 = 2d = -1
Sn = (a1 + an) * n / 2 = (a3 - 2d + an) * n / 2 = (an + 1) * n / 2 = 1/3(an - 1)
an^2 -1 = 2 / 3n
……