JEE Main & Advanced Mathematics Functions Question Bank Functions

  • question_answer
    If \[f(x+ay,\ x-ay)=axy\], then \[f(x,\ y)\] is equal to            [AMU 2001]

    A)            xy

    B)            \[{{x}^{2}}-{{a}^{2}}{{y}^{2}}\]

    C)            \[\frac{{{x}^{2}}-{{y}^{2}}}{4}\]

    D)            \[\frac{{{x}^{2}}-{{y}^{2}}}{{{a}^{2}}}\]

    Correct Answer: C

    Solution :

               Given \[f(x+ay,\,x-ay)=axy\]                           ?..(i)                 Let \[x+ay=u\] and \[x-ay=v\]            Then \[x=\frac{u+v}{2}\] and \[y=\frac{u-v}{2a}\]            Substituting the value of x and y in (i), we obtain                    \[f(u,v)=\frac{{{u}^{2}}-{{v}^{2}}}{4}\] Þ \[f(x,\,y)=\frac{{{x}^{2}}-{{y}^{2}}}{4}\].


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