等差数列an的公差为d=2,若a1,a3,a4成等比数列,求a2.
问题描述:
等差数列an的公差为d=2,若a1,a3,a4成等比数列,求a2.
答
a3=a1+2d,
a4=a1+3d,
由题意可得:(a1+2d)^2=a1(a1+3d)
化简得a1d+4d^2=0
因为d=2,所以a1=-4d=-8
所以a2=a1+d=-6