# math

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The equation
4x^2 +9y^2 -36 = 0
represents an ellipse in standard position.
Find the coordinates of the foci and the equations of the directrices.
The Foci are (+-f, 0) where f>0
I have worked out f=sqrt(5)

not sure how to work out the directrices (d)

• math -

In standard form,

x^2/9 + y^2/4 = 1

a = 3
b = 2
c = √5

You are correct that the foci are at (±√5,0)

The directrices are at x = ±a^2/c = ±9/√5

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