Maths-
General
Easy

Question

If the harmonic mean between a and b is H, then fraction numerator H plus a over denominator H minus a end fraction plus fraction numerator H plus b over denominator H minus b end fraction equals

  1. 4    
  2. 2    
  3. 1    
  4. a+b    

The correct answer is: 2


    text Putting  end text H equals fraction numerator 2 a b over denominator a plus b end fraction
    fraction numerator H plus a over denominator H minus a end fraction plus fraction numerator H plus b over denominator H minus b end fraction equals fraction numerator 2 open parentheses H to the power of 2 end exponent minus a b close parentheses over denominator left parenthesis H minus a right parenthesis left parenthesis H minus b right parenthesis end fraction equals fraction numerator 2 open square brackets fraction numerator 4 a b over denominator left parenthesis a plus b right parenthesis to the power of 2 end exponent end fraction minus a b close square brackets over denominator open square brackets fraction numerator 4 a b over denominator left parenthesis a plus b right parenthesis to the power of 2 end exponent end fraction minus a b close square brackets end fraction equals 2
    text Trick: Let  end text a equals 1 comma H equals fraction numerator 1 over denominator 2 end fraction text  and  end text b equals fraction numerator 1 over denominator 3 end fraction text  . Then end text
    fraction numerator H plus a over denominator H minus a end fraction plus fraction numerator H plus b over denominator H minus b end fraction equals fraction numerator 3 divided by 2 over denominator negative 1 divided by 2 end fraction plus fraction numerator 5 divided by 6 over denominator 1 divided by 6 end fraction equals 2

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