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Question

Let f:RR be the function f(x)=(xa1)(xa2)+(xa2)(xa3)+(xa3)(xa1) with a1,a2,a3R. Then f(x)0 if and only if

A
At least two of a1,a2,a3 are equal
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B
a1=a2=a3
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C
a1,a2,a3 are all distinct
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D
a1,a2,a3 are all positive and distinct
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Solution

The correct option is A a1=a2=a3
f(x)=(xx1)+(xx2)+(xx3)f(x)=x22(a1+a2+a2)x+a1a2+a2a2+a2a2f(x)0D04(a1+a2+a2)24×1×(a1a2+a2a2+a2a2)0a21+a22+a23a1a2a2a2a2a2012[(a1a2)2+(a2a3)2+(a3a1)2]0
All the terms are perfect square so they cant be less than 0
(a1a2)2+(a2a3)2+(a3a1)2=0
which is only possible when a1=a2=a3

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