Maths-
General
Easy

Question

If the sum to infinity of the series 1 plus 2 r plus 3 r to the power of 2 end exponent plus 4 r to the power of 3 end exponent plus midline horizontal ellipsis is 9/4, then value of r is

  1. 1/2   
  2. 1/3  
  3. 1/4   
  4. None of these  

The correct answer is: 1/3


    S equals 1 plus 2 r plus 3 r to the power of 2 end exponent plus 4 r to the power of 3 end exponent plus horizontal ellipsis
    r S equals r plus 2 r to the power of 2 end exponent plus 3 r to the power of 3 end exponent plus 4 r to the power of 4 end exponent plus horizontal ellipsis
    rightwards double arrow open parentheses 1 minus r close parentheses S equals 1 plus r plus r to the power of 2 end exponent plus r to the power of 3 end exponent plus horizontal ellipsis
    equals fraction numerator 1 over denominator 1 minus r end fraction
    rightwards double arrow S equals fraction numerator 1 over denominator open parentheses 1 minus r close parentheses to the power of 2 end exponent end fraction
    Given, S equals 9 divided by 4 blank rightwards double arrow fraction numerator 1 over denominator open parentheses 1 minus r close parentheses to the power of 2 end exponent end fraction equals 9 divided by 4
    rightwards double arrow 1 minus r equals plus-or-minus fraction numerator 2 over denominator 3 end fraction
    rightwards double arrow r equals 1 divided by 3 or 5 divided by 3
    Hence, r equals 1 divided by 3 as 0 less than open vertical bar r close vertical bar less than 1

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