Manipal Engineering Manipal Engineering Solved Paper-2008

  • question_answer
    In\[\Delta ABC,\,\,{{(a-b)}^{2}}{{\cos }^{2}}\frac{C}{2}+{{(a+b)}^{2}}{{\sin }^{2}}\frac{C}{2}\]is equal to

    A) \[{{a}^{2}}\]                                      

    B) \[{{b}^{2}}\]

    C) \[{{c}^{2}}\]                       

    D)         None of these

    Correct Answer: C

    Solution :

    \[(a-b){{\cos }^{2}}\frac{C}{2}+{{(a+b)}^{2}}{{\sin }^{2}}\frac{C}{2}\]                 \[=({{a}^{2}}+{{b}^{2}}-2ab){{\cos }^{2}}\frac{C}{2}\]                                 \[+({{a}^{2}}+{{b}^{2}}+2ab){{\sin }^{2}}\frac{C}{2}\]                 \[={{a}^{2}}+{{b}^{2}}+2ab\left( {{\sin }^{2}}\frac{C}{2}-{{\cos }^{2}}\frac{C}{2} \right)\]                 \[={{a}^{2}}+{{b}^{2}}-2ab\cos C\]                 \[={{a}^{2}}+{{b}^{2}}-({{a}^{2}}+{{b}^{2}}-{{c}^{2}})\]                 \[={{c}^{2}}\]


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