8th Class Mathematics Algebraic Expressions Question Bank Algebraic Expressions & Identities

  • question_answer
    If \[a=\frac{\sqrt{5}-\sqrt{3}}{\sqrt{5}+\sqrt{3}}\] and \[b=\frac{\sqrt{5}+\sqrt{3}}{\sqrt{5}-\sqrt{3}}\] then the value of \[\frac{{{a}^{2}}-ab+{{b}^{2}}}{{{a}^{2}}+ab+{{b}^{2}}}\]=?

    A)  \[\frac{61}{63}\]          

    B)  \[\frac{67}{65}\]

    C)  \[\frac{65}{63}\]                                  

    D)  \[\frac{69}{67}\]

    Correct Answer: A

    Solution :

    (a): again ab = 1 \[a={{\left( \frac{\sqrt{5}-\sqrt{3}}{2} \right)}^{2}};b={{\left( \frac{\sqrt{5}+\sqrt{3}}{2} \right)}^{2}}\] Expression =\[\frac{{{\left( a-b \right)}^{2}}+ab}{{{\left( a+b \right)}^{2}}+ab}\] Expression=\[\frac{{{\left( -2\times \sqrt{15} \right)}^{2}}+1}{{{8}^{2}}-1}=\frac{61}{63}\left\{ \begin{align}   & as(a-b)=-2\sqrt{15} \\  & and\,(a+b)=8 \\ \end{align} \right\}\]      


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