Find Equation of Elipse with b=3, M(-2SQRT(5),2)

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SUMMARY

The equation of the ellipse with a semi-minor axis \( b = 3 \) and a point \( M(-2\sqrt{5}, 2) \) on it is derived using the standard form \( \frac{x^2}{a^2} + \frac{y^2}{b^2} = 1 \). The calculation shows that \( a = 2 \), leading to the equation \( \frac{x^2}{4} + \frac{y^2}{9} = 1 \). However, the correct form of the equation, as per the textbook, is \( 9x^2 + 36y^2 = 324 \). The error in the initial calculation stemmed from not multiplying the constant term correctly, which should have been \( 180 + 4a^2 = 9a^2 \).

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Homework Statement


Find equation of elips if b=3 and point M(-2SQRT(5), 2) is on elipse.

Homework Equations



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

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?
 
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This bit:

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

20 + 4a^2 = 9a^2

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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