Calculating hyperbola equation from directrix and foci

wduk

New member
Joined
Dec 11, 2016
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46
Hello

I am a bit stuck with calculating my hyperbola from some properties. Namely Focus (±ae,0) of (4,0) and directrix (±a/e) x = 3.

This is my current calculation, but the problem is my b^2 is negative which doesn't seem right.

1) ae = 4
2) a/e = 3

Multiply 1 & 2: a^2 = 12, so a = 6

solve e using a for equation 1:

6e = 4 so e = 4/6

Now to solve b^2 for my equation i get:

b^2 = 6^2((4/6)^2 - 1) = -20

Which would give an equation for my hyperbola as:

x^2/12 + y^2/20 = 1

But this is an ellipse so where have i gone wrong?
 
Nevermind I'm a total moron, i halved the value of a instead of using square root of a! Silly me!
 
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