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Basic Elipse problem

  1. Mar 21, 2012 #1
    1. The problem statement, all variables and given/known data
    Find equation of elips if b=3 and point M(-2SQRT(5), 2) is on elipse.

    2. Relevant equations

    x^2/a^2 + y^2/b^2 = 1

    3. The attempt at a solution
    x^2/a^2 + y^2/b^2 = 1

    20/a^2 + 4/9 = 1

    20/a^2 + 4/9 = 1 / * a^2

    20 + 4a^2/9 = a^2

    20 + 4^2 = 9a^2

    20= 5a^2

    a = 2

    so equation of elipse is x^2/4 + y^2/9 = 1

    But answer at the end of a book is 9x^2 + 36y^2 = 324

    Which one is correct?
     
  2. jcsd
  3. Mar 21, 2012 #2

    Curious3141

    User Avatar
    Homework Helper

    This bit:

    is wrong, because you should multiply the 20 by 9 too. Equation should become

    180 + 4a^2 = 9a^2, which would give you the right solution.
     
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