Mathematics
Functions
Easy

Question

For real x,text  let  end text f left parenthesis x right parenthesis equals x cubed plus 5 x plus 1 comma text end textthen

  1. f text  is onto on  end text R text  but not one-one  end text
  2. f text  is one-one and onto on  end text R
  3. text  fis neither one-one nor onto on  end text R
  4. f text  is one-one but not onto on  end text R

The correct answer is: f text  is one-one and onto on  end text R


    text  The function  end text f colon R not stretchy rightwards arrow R text  is defined as  end text f left parenthesis x right parenthesis equals x cubed plus 5 x plus 1
text  Let  end text y element of R text  then  end text y equals x cubed plus 5 x plus 1 not stretchy rightwards double arrow x cubed plus 5 x plus 1 minus y equals 0
text  As a polynomial of odd degree has always at least one real rot, corresponding end text
text to any  end text y element of text  co-domain.  end text
text  So  end text straight there exists text  some  end text x element of text  domain such that  end text f left parenthesis x right parenthesis equals y. text  Hence  end text f text  is onto.  end text
text  Also  end text f text  is continuous on  end text R text  , because it's a polynomial function. end text
text Now  end text f to the power of straight prime times left parenthesis x right parenthesis equals 3 x squared plus 5 greater than 0 text  . end text
therefore f text  is strictly increasing. Hence  end text f text  is one-one also.   end text

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