Find foci of the equation \(x^2 + 2x – 4y^2 + 8y – 7 = 0\)
Step-by-step Solution:
The given equation is:
\[
x^2 + 2x - 4y^2 + 8y - 7 = 0
\]
Step 1: Identify the Conic Section
Rewriting the equation:
\[
x^2 + 2x - 4(y^2 - 2y) = 7
\]
Since the coefficients of \(x^2\) and \(y^2\) have opposite signs, it represents a hyperbola.
Step 2: Complete the Square
Completing the square for \(x\):
\[
x^2 + 2x = (x + 1)^2 - 1
\]
Completing the square for \(y\):
\[
y^2 - 2y = (y - 1)^2 - 1
\]
Multiplying by \(-4\):
\[
-4(y^2 - 2y) = -4((y - 1)^2 - 1) = -4(y - 1)^2 + 4
\]
Substituting these back into the equation:
\[
(x + 1)^2 - 1 - 4(y - 1)^2 + 4 = 7
\]
\[
(x + 1)^2 - 4(y - 1)^2 + 3 = 7
\]
\[
(x + 1)^2 - 4(y - 1)^2 = 4
\]
\[
\frac{(x + 1)^2}{4} - \frac{(y - 1)^2}{1} = 1
\]
Step 3: Find the Foci
For the standard hyperbola equation:
\[
\frac{(x - h)^2}{a^2} - \frac{(y - k)^2}{b^2} = 1
\]
We compare:
\( h = -1 \), \( k = 1 \)
\( a^2 = 4 \Rightarrow a = 2 \)
\( b^2 = 1 \Rightarrow b = 1 \)
The focal distance is given by:
\[
c^2 = a^2 + b^2 = 4 + 1 = 5
\]
\[
c = \sqrt{5}
\]
Since it is a horizontally oriented hyperbola, the foci are at:
\[
(x + 1) = \pm c
\]
\[
(x, y) = (-1 \pm \sqrt{5}, 1)
\]
Final Answer:
\[
{(-1 + \sqrt{5}, 1) \text{ and } (-1 - \sqrt{5}, 1)}
\]