Let a>0,a≠1. Then the set S of all positive real numbers b satisfying (1+a2)(1+b2)=4ab is
A
an empty set
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B
a singleton set
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C
a finite set containing more than one element
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D
(0,∞)
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Solution
The correct option is A an empty set Given (1+a2)(1+b2)=4ab ⇒1+a2+b2+a2b2=4ab ⇒a2+b2−2ab+a2b2+1−2ab=0 ⇒(a−b)2+(ab−1)2=0 This is only possible when, (a−b)=0 and (ab−1)=0 ∴a=b and ∴ab=1 ⇒a2=1 ⇒a=1 but this is not possible as given in the question. Hence, S will be an empty set.