BCECE Engineering BCECE Engineering Solved Paper-2015

  • question_answer
    1f |a| < 1 and |b| < 1, then the sum of the series \[a(a+b)+{{a}^{2}}({{a}^{2}}+{{b}^{2}})+{{a}^{3}}({{a}^{3}}+{{b}^{3}})+...\] is    

    A) \[\frac{a}{1-a}+\frac{ab}{1-ab}\]             

    B) \[\frac{{{a}^{2}}}{1-{{a}^{2}}}+\frac{ab}{1-ab}\]

    C) \[\frac{b}{1-b}+\frac{a}{1-a}\]                 

    D) \[\frac{{{b}^{2}}}{1-{{b}^{2}}}+\frac{ab}{1-ab}\]

    Correct Answer: B

    Solution :

    We have, \[|a|<1,|b|<1\] \[\therefore \]  \[|ab|=|a|b|<1\] Now \[a(a+b)+{{a}^{2}}({{a}^{2}}+{{b}^{2}})+{{a}^{3}}({{a}^{3}}+{{b}^{3}})+...\] \[=[({{a}^{2}}+{{a}^{4}}+{{a}^{6}}+...)]+[\{ab+(a{{b}^{2}})+(a{{b}^{3}})+...\}]\]  \[=\frac{{{a}^{2}}}{1-{{a}^{2}}}+\frac{ab}{1-ab}\]


You need to login to perform this action.
You will be redirected in 3 sec spinner