CET Karnataka Engineering CET - Karnataka Engineering Solved Paper-2008

  • question_answer
    If a and b are unit vectors and \[|\vec{a}+\vec{b}|=1,\] then\[|\vec{a}-\vec{b}|=1,\] is equal to

    A)  \[\sqrt{2}\]                                       

    B)  \[1\]

    C)  \[\sqrt{5}\]                                       

    D)  \[\sqrt{3}\]

    Correct Answer: D

    Solution :

    Given,        \[|\vec{a}+\vec{b}|=1\] \[\therefore \]  \[|\vec{a}+\vec{b}{{|}^{2}}={{1}^{2}}\] \[\Rightarrow \]               \[|\vec{a}|+|\vec{b}{{|}^{2}}+2|\vec{a}||\vec{b}|=1\] \[\Rightarrow \]                               \[2|\vec{a}||\vec{b}|=1-(1+1)\] \[\Rightarrow \]                               \[2|\vec{a}||\vec{b}|=-1\]                         ?.(1) Now.    \[|\vec{a}-\vec{b}{{|}^{2}}=|\vec{a}{{|}^{2}}+|\vec{b}{{|}^{2}}-2|\vec{a}||\vec{b}|\]                 \[={{1}^{2}}+{{1}^{2}}-(-1)=3\]   [form Eq. (i)] \[\Rightarrow \]               \[|\vec{a}-\vec{b}|=\sqrt{3}\]


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