设出公比并表示出a2=a1•q,a3=a1•q2,a4=a1•q3,然后求出公比,进而得出a1,从而求出a2,a3,a4的值.
【解析】
设公比是q,则a2=a1•q,a3=a1•q2,a4=a1•q3
∴a1-a2=a1-a1•q=a1(1-q)=36 ①
a3-a4=a1•q2-a1•q3=a1•q2•(1-q)=4 ②
=q2=
解得:q=±
(1)当q=时,(1-)a1=36 解得:a1=54,则a2=18,a3=6,a4=2
(2)当q=-时,[1-(-)]a1=36,解得a1=27,则a2=-9,a3=3,a4=-1
终上所述:
a1,a2,a3,a4的值为:a1=54,a2=18,a3=6,a4=2
或:a1=27,a2=-8,a3=3,a4=-1