已知a1,a2,a3,a4成等比数列,且a1=a2+36,a3=a4+4,求a1,a2,a3,a4.
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已知a1,a2,a3,a4成等比数列,且a1=a2+36,a3=a4+4,求a1,a2,a3,a4.
![已知a1,a2,a3,a4成等比数列,且a1=a2+36,a3=a4+4,求a1,a2,a3,a4.](/uploads/image/z/6744984-24-4.jpg?t=%E5%B7%B2%E7%9F%A5a1%EF%BC%8Ca2%EF%BC%8Ca3%EF%BC%8Ca4%E6%88%90%E7%AD%89%E6%AF%94%E6%95%B0%E5%88%97%EF%BC%8C%E4%B8%94a1%3Da2%2B36%EF%BC%8Ca3%3Da4%2B4%EF%BC%8C%E6%B1%82a1%EF%BC%8Ca2%EF%BC%8Ca3%EF%BC%8Ca4%EF%BC%8E)
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
②
①=q2=
1
9
解得:q=±
1
3
(1)当q=
1
3时,(1-
1
3)a1=36 解得:a1=54,则a2=18,a3=6,a4=2
(2)当q=-
1
3时,[1-(-
1
3)]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
∴a1-a2=a1-a1•q=a1(1-q)=36 ①
a3-a4=a1•q2-a1•q3=a1•q2•(1-q)=4 ②
②
①=q2=
1
9
解得:q=±
1
3
(1)当q=
1
3时,(1-
1
3)a1=36 解得:a1=54,则a2=18,a3=6,a4=2
(2)当q=-
1
3时,[1-(-
1
3)]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
已知a1,a2,a3,a4成等比数列,且a1=a2+36,a3=a4+4,求a1,a2,a3,a4.
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