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Question

text If  end text theta element of left square bracket 05 pi right square bracket text  and  end text r element of R text  such that  end text 2 s i n invisible function application theta equals r to the power of 4 end exponent minus 2 r to the power of 2 end exponent plus 3 then the maximum number of values of the pair left parenthesis r comma theta right parenthesis is

  1. 8    
  2. 10    
  3. 6    
  4. None of these    

The correct answer is: 6


    Hint: For real values of x, the discriminant of a quadratic equation is greater than or equal to zero.
    text Given equation is  end text r to the power of 4 end exponent minus 2 r to the power of 2 end exponent plus 3 minus 2 s i n invisible function application theta equals 0 text  . end text
    text The above equation is quadratic in  end text r to the power of 2 end exponent text  . For the real values of  end text r to the power of 2 end exponent text  , end text text discriminant > 0. Therefore, end text
    table row cell 4 minus 4 left parenthesis 3 minus 2 s i n invisible function application theta right parenthesis greater or equal than 0 end cell row cell rightwards double arrow 1 minus 3 plus 2 s i n invisible function application theta greater or equal than 0 end cell row cell rightwards double arrow negative 2 plus 2 s i n invisible function application theta greater or equal than 0 rightwards double arrow s i n invisible function application theta greater or equal than 1 end cell end table
    text That is, the above inequality holds only at  end text sin invisible function application theta equals 1 text  . end text
    text Possible value of  end text theta equals fraction numerator pi over denominator 2 end fraction comma 2 pi plus fraction numerator pi over denominator 2 end fraction comma 4 pi plus fraction numerator pi over denominator 2 end fraction
    text And  end text r to the power of 4 end exponent minus 2 r to the power of 2 end exponent plus 3 minus 2 equals 0 rightwards double arrow open parentheses r to the power of 2 end exponent minus 1 close parentheses to the power of 2 end exponent equals 0 rightwards double arrow r equals negative 11
    text Hence, number of ordered pair of  end text left parenthesis r comma theta right parenthesis text  is  end text 6.

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