Maths-
General
Easy

Question

If S subscript n end subscript denotes the sum of first n terms of an A.P. whose first term is a and fraction numerator S subscript n x end subscript over denominator S subscript x end subscript end fraction is independent of x, then S subscript p end subscript equals

  1. p to the power of 3 end exponent   
  2. p to the power of 2 end exponent a   
  3. p a to the power of 2 end exponent   
  4. a to the power of 3 end exponent  

The correct answer is: p to the power of 2 end exponent a


    fraction numerator S subscript n x end subscript over denominator S subscript x end subscript end fraction equals fraction numerator fraction numerator n x over denominator 2 end fraction left square bracket 2 a plus left parenthesis n x minus 1 right parenthesis d right square bracket over denominator fraction numerator x over denominator 2 end fraction left square bracket 2 a plus left parenthesis x minus 1 right parenthesis d right square bracket end fraction
    equals fraction numerator n left square bracket open parentheses 2 a minus d close parentheses plus n x d right square bracket over denominator open parentheses 2 a minus d close parentheses plus x d end fraction
    For fraction numerator S subscript n x end subscript over denominator S subscript x end subscript end fraction to be independent of x,
    2 a minus d equals 0 blank rightwards double arrow 2 a equals d
    Now, S subscript P end subscript equals fraction numerator p over denominator 2 end fraction open square brackets 2 a plus open parentheses p minus 1 close parentheses d close square brackets equals p to the power of 2 end exponent a

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