Maths-
General
Easy

Question

If open vertical bar a close vertical bar less than 1 and open vertical bar b close vertical bar less than 1, then the sum of the series 1 plus open parentheses 1 plus a close parentheses b plus open parentheses 1 plus a plus a to the power of 2 end exponent close parentheses b to the power of 2 end exponent plus open parentheses 1 plus a plus a to the power of 2 end exponent plus a to the power of 3 end exponent close parentheses b to the power of 3 end exponent plus horizontal ellipsis is

  1. fraction numerator 1 over denominator open parentheses 1 minus a close parentheses left parenthesis 1 minus b right parenthesis end fraction   
  2. fraction numerator 1 over denominator open parentheses 1 minus a close parentheses left parenthesis 1 minus a b right parenthesis end fraction  
  3. fraction numerator 1 over denominator open parentheses 1 minus b close parentheses left parenthesis 1 minus a b right parenthesis end fraction   
  4. fraction numerator 1 over denominator open parentheses 1 minus a close parentheses open parentheses 1 minus b close parentheses left parenthesis 1 minus a b right parenthesis end fraction  

The correct answer is: fraction numerator 1 over denominator open parentheses 1 minus b close parentheses left parenthesis 1 minus a b right parenthesis end fraction


    We have,
    1 plus open parentheses 1 plus a close parentheses b plus open parentheses 1 plus a plus a to the power of 2 end exponent close parentheses b to the power of 2 end exponent plus open parentheses 1 plus a plus a to the power of 2 end exponent plus a to the power of 3 end exponent close parentheses b to the power of 3 end exponent plus horizontal ellipsis infinity blank
    equals not stretchy sum from n equals 1 to infinity of left parenthesis 1 plus a plus a to the power of 2 end exponent plus horizontal ellipsis plus a to the power of n minus 1 end exponent right parenthesis b to the power of n minus 1 end exponent
    equals not stretchy sum from n equals 1 to infinity of open parentheses fraction numerator 1 minus a to the power of n end exponent over denominator 1 minus a end fraction close parentheses b to the power of n minus 1 end exponent
    equals not stretchy sum from n equals 1 to infinity of fraction numerator b to the power of n minus 1 end exponent over denominator 1 minus a end fraction minus not stretchy sum from n equals 1 to infinity of fraction numerator a to the power of n end exponent b to the power of n minus 1 end exponent over denominator 1 minus a end fraction
    equals fraction numerator 1 over denominator 1 minus a end fraction not stretchy sum from n equals 1 to infinity of b to the power of n minus 1 end exponent minus fraction numerator a over denominator 1 minus a end fraction not stretchy sum from n equals 1 to infinity of open parentheses a b close parentheses to the power of n minus 1 end exponent
    equals fraction numerator 1 over denominator 1 minus a end fraction open square brackets 1 plus b plus b to the power of 2 end exponent plus horizontal ellipsis infinity close square brackets minus fraction numerator a over denominator 1 minus a end fraction left square bracket 1 plus a b plus open parentheses a b close parentheses to the power of 2 end exponent plus horizontal ellipsis infinity right square bracket
    equals fraction numerator 1 over denominator 1 minus a end fraction cross times fraction numerator 1 over denominator 1 minus b end fraction minus fraction numerator a over denominator open parentheses 1 minus a close parentheses left parenthesis 1 minus a b right parenthesis end fraction
    equals fraction numerator 1 over denominator open parentheses 1 minus a b close parentheses left parenthesis 1 minus b right parenthesis end fraction

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