12th Class Mathematics Sample Paper Mathematics Sample Paper-3

  • question_answer
    Let \[f,\,g:R\to R\] be two functions defined as \[f(x)=|x|+x,\] \[g(x)=|x|-x,\] \[\forall x\in R.\] Then, find fog and gof.

    Answer:

    Given, \[f(x)=|x|+x,\] which can be redefined as                         \[f(x)=\left\{ \begin{align}   & 2x,\,\,\text{if}\,\,x\ge 0 \\  & 0,\,\,\text{if}\,\,x<0 \\ \end{align} \right.\]             and       \[g(x)=\,\,|x|-x\]             \[\Rightarrow \]   \[g(x)=\left\{ \begin{align}   & 0,\,\,\text{if}\,\,x\ge 0 \\  & -2x,\,\,\text{if}\,\,x<0 \\ \end{align} \right.\]             Now, gof gets defined as             For \[x\ge 0,\] \[(gof)\,(x)=g(f(x))=g(2x)=0\]             and for \[x<0,\] \[(gof)\,(x)=g(f(x))=g(0)=0\] Consequently, we have (gof)(x) = 0, \[\forall x\in R\] Similarly, fog gets defined as For \[x\ge 0,\] (fog)(x) = f(g(x)) = f(0) = 0 and for \[x\ge 0,\] \[(fog)(x)=f(g(x))=f(-2x)=-\,4x\]             i.e. \[(fog)\,(x)=\left\{ \begin{matrix}    0, & x\ge 0  \\    -\,4x, & x<{{0}^{.}}  \\ \end{matrix} \right.\]


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