Simultaneous Equations

• kkhing

  19 Jan 12, 14:39

  Hi,

  Can someone help to solve this question. This is my daughter sec 3 Maths question. I tried but couldn't get an anser. Thanks.

  2x+5y=6
  4x^2+16y^2=20x+4y-19

   

  

• Summer hill

  19 Jan 12, 17:08

  2x = 6 - 5y and so 20x = 60 - 50y
  and so 4x^2 = 36 + 25y^2 - 60y
  sub for x
  
  36 + 25y^2 - 60y + 16y^2 = 60 - 50y + 4y -19
  36 + 41y^2 - 60y = 41 - 46y
  -5 + 41y^2 - 14y = 0
  41y^2 - 14y - 5 = 0
  
  y = (14 +/- sqrt(196 + 820))/82
  y = (14 + 31.87)/82 or (14 - 31.87)/82
  y = 0.559 or - 0.218
  and so when y = 0.559
  2x + 2.795 = 6
  x = (6 - 2.795)/2 = 1.603
  or when y = - 0.218
  2x - 1.09 = 6
  x = (6 + 1.09)/2 = 3.545
  Answers are
  x = 3.545, y = -0.218
  or
  x = 1.603, y = 0.559

• Summer hill

  20 Jan 12, 14:17

  er tell me if it is correct or not?

• wee_ws

  20 Jan 12, 15:03

  Hi,

    Backward substitution will just do :)

    Thanks.

  Cheers,
  Wen Shih