Upper sec Emaths prob

• Jundead

  25 Jan 11, 01:23

  Hi, I have encounter such problem while doing the TYS. I have found similar topic in this forum, but the thread has expired.

  The link is http://politics.sgforums.com/forums/2297/topics/331571

  Does anyone know how to solve part j with respect to the question from the link?

  
  Many thks!

• eagle

  26 Jan 11, 13:20

  Hi,

  Set x² - x - 5 = 1/x

  Rearrange and you will get

  x³ - x² - 5x = 1

  x³ - x² - 5x - 1 = 0

  That is your cubic equation.

   

  Do continue to ask if you are still unsure. :)

   

• Jundead

  26 Jan 11, 18:26

  Thks eagle for the solution.

  
  But can you explain why u set the two eqns equal to one another?

• Forbiddensinner

  26 Jan 11, 21:11

  Originally posted by Jundead:

  Thks eagle for the solution.

  
  But can you explain why u set the two eqns equal to one another?

  At the intersections, the y-values will be the same, so you can equate the equations to each other. Solving the new equation will give you the x-value of the intersections, but that is not what we want to do here.

   

• Bluehawk156

  26 Jan 11, 21:12

  what happened to the 1?

  Using your equation, x²- x - 5 = 1/x

  you're supposed to get x³ - x² - 5x-1 = 0

  I mean like aren't you multiplying both sides by x?

• wee_ws

  27 Jan 11, 06:12

  Hi Bluehawk156,

    Yes, you are correct. Thanks.

  Cheers,
  Wen Shih

• eagle

  27 Jan 11, 11:26

  Yup, the 1 should be there. My bad.

  Changed it. Thanks.