JEE Main & Advanced Mathematics Functions Question Bank Functions

  • question_answer
    Let \[f(x)=ax+b\] and \[g(x)=cx+d,\ a\ne 0,\ c\ne 0\]. Assume \[a=1,\ b=2\]. If \[(fog)(x)=(gof)(x)\] for all x, what can you say about c and d [AMU 2000]

    A)                    c and d both arbitrary

    B)            \[c=1,\ d\] arbitrary

    C)            c arbitrary, \[d=1\]

    D)            \[c=1,\ d=1\]

    Correct Answer: B

    Solution :

               \[(fog)(x)=f(g(x))=a(cx+d)+b\]                    and \[(gof)(x)=g(f(x))=c(ax+b)+d\]            Given that, \[(fog)(x)=(gof)(x)\] and at \[a=1,\,b=2\]            Þ \[\underset{x\to 0}{\mathop{\lim }}\,\,\,3\,\frac{\tan 3x}{3x}+\underset{x\to 0}{\mathop{\lim }}\,\cos x=3+1=4\] Þ \[c=1\] and d is arbitrary.


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