已知等差数列{an}的公差为2,若a1,a3,a4成等比数列,则a2=?

题目

已知等差数列{an}的公差为2,若a1,a3,a4成等比数列,则a2=?
等比数列{an}中,a1+a3=10,a4+a6=5/4,则数列{an}的通项公式为?

答案

已知等差数列{an}的公差为2,若a1,a3,a4成等比数列,则a2=?a1 a3 a4成等比数列a3²=a1*a4(a1+2d)²=a1(a1+3d)a1²+4a1d+4d²=a1²+3a1da1d=-4d²所以a1=-4d=-8a2=a1+d=-6等比数列{an}中,a1+a3...