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已知实数a,b,c满足abc=-1,a+b+c=4,
a
a2−3a−1
+
b
b2−3b−1
+
c
c2−3c−1
4
9
,则a2+b2+c2=______.
人气:433 ℃ 时间:2019-12-05 16:29:30
解答
∵abc=-1,a+b+c=4,
∴a2-3a-1=a2-3a+abc=a(bc+a-3)=a(bc-b-c+1)=a(b-1)(c-1),
a
a2−3a−1
=
1
(b−1)(c−1)

同理可得:
b
b2−3b−1 
=
1
(a−1)(c−1)
c
c2−3c−1
=
1
(a−1)(b−1)

a
a2−3a−1
+
b
b2−3b−1 
+
c
c2−3c−1
=
4
9

1
(b−1)(c−1)
+
1
(a−1)(c−1)
+
1
(a−1)(b−1)
=
4
9

(a−1)+(b−1)+(c−1)
(a−1)(b−1)(c−1)
=
4
9
,即
4
9
(a-1)(b-1)(c-1)=(a-1)+(b-1)+(c-1),
整理得:
4
9
(abc-ab-ac-bc+a+b+c-1)=a+b+c-3,
将abc=-1,a+b+c=4代入得:ab+bc+ac=-
1
4

则a2+b2+c2=(a+b+c)2-2(ab+bc+ac)=
33
2

故答案为:
33
2
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