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Question

if cos left parenthesis x minus y right parenthesis, cos x and cos left parenthesis x plus y right parenthesis are in HP, then cos x times s e c invisible function application fraction numerator y over denominator 2 end fraction is equal to

  1. square root of 2    
  2. negative square root of 2    
  3. plus-or-minus square root of 2    
  4. text None of these end text    

The correct answer is: plus-or-minus square root of 2


    Given cos left parenthesis x minus y right parenthesis, cos x and cosleft parenthesis x plus y right parenthesis are in HP. So
    fraction numerator 2 over denominator cos invisible function application x end fraction equals fraction numerator 1 over denominator c o s invisible function application left parenthesis x minus y right parenthesis end fraction plus fraction numerator 1 over denominator c o s invisible function application left parenthesis x plus y right parenthesis end fraction
    rightwards double arrow fraction numerator 2 over denominator cos invisible function application x end fraction equals fraction numerator 2 c o s invisible function application x times c o s invisible function application y over denominator c o s to the power of 2 end exponent invisible function application x minus s i n to the power of 2 end exponent invisible function application y end fraction
    rightwards double arrow c o s to the power of 2 end exponent invisible function application x minus s i n to the power of 2 end exponent invisible function application y equals c o s to the power of 2 end exponent invisible function application x times c o s invisible function application y
    rightwards double arrow c o s to the power of 2 end exponent invisible function application x times 2 s i n to the power of 2 end exponent invisible function application fraction numerator y over denominator 2 end fraction equals 4 s i n to the power of 2 end exponent invisible function application fraction numerator y over denominator 2 end fraction times c o s to the power of 2 end exponent invisible function application fraction numerator y over denominator 2 end fraction
    rightwards double arrow c o s invisible function application x times s e c invisible function application fraction numerator y over denominator 2 end fraction equals plus-or-minus square root of 2

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