Rewrite hyperbola $Ax^2+Bxy+Dx+Ey+F=0$ into standard form

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The general conic section is given by $$Ax^2+Bxy+Cy^2+Dx+Ey+F=0.$$

In the case where $B\neq0$, $C=0$, we have $$Ax^2+Bxy+Dx+Ey+F\overset{1}=0,$$ with $B^2-4AC=B^2>0$, so that this is a hyperbola.

How do we rewrite $\overset{1}=$ into the standard (or canonical) form?

(Ideally, please just show the algebra and do not make use of other results from conic sections without also proving them.)

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Basically, the idea is to rotate your axes so that the given curve is a hyperbola in the usual sense. In order to do this, we let

$$x=X\cos \theta+Y\sin\theta \; \; \; \; \; \; \; \; \; \; \; \; \; y=-X\sin\theta+Y\cos\theta$$

where $(X,Y)$ are the coordinates in the rotated axes.

For ease of notation, I shall define

$$c=\cos\theta \; \; \; \; \; \; \; \; \; \; \; \; \; s = \sin\theta$$

Plugging this in, we get

$$(Ac^2-Bsc)X^2+(2Asc+Bc^2-Bs^2)XY+(As^2+Bsc)Y^2+(Dc-Es)X+(Ds+Ec)Y=-F$$

With a hyperbola, we want to make sure that the $XY$ term disappears. i.e., we want to choose $\theta$ such that the coefficient of $XY$, which is $2Asc+Bc^2-Bs^2$, is $0$.

\begin{align} \ & 2Asc+Bc^2-Bs^2=0 \\ \ \implies & A\sin(2\theta)+B\cos(2\theta)=0 \\ \ \implies & \theta=-\frac 12 \tan^{-1} \frac BA \end{align}

With this particular value of $\theta$, you work out what $\sin \theta$ and $\cos \theta$ are, and plug them back into the equation above to get something in the from of $$PX^2+QY^2+RX+SY=-F$$

which implies that

$$P \biggl(X+\frac{R}{2P} \biggr)^2-Q \biggl(Y+\frac{S}{2Q} \biggr)^2=\frac{R^2}{4P}+\frac{S^2}{4Q}-F$$

where $P,Q$ should be positive.

Then, via a shift in the axes and some rearrangements, we get the required hyperbola.


Probably easier to understand with an example.

Say we have the equation

$$x^2-\sqrt 3xy+\sqrt 3x+y-\frac 73=0$$

Then as before, we let

$$x=X\cos \theta+Y\sin\theta \; \; \; \; \; \; \; \; \; \; \; \; \; y=-X\sin\theta+Y\cos\theta$$

$$c=\cos\theta \; \; \; \; \; \; \; \; \; \; \; \; \; s = \sin\theta$$

Plug this in to get

$$(c^2+\sqrt 3sc)X^2+(2sc-\sqrt 3c^2+\sqrt 3s^2)XY+(s^2 - \sqrt 3sc)Y^2+(\sqrt 3c-s)X+(\sqrt 3s+c)Y=\frac 73$$

We want to chose $\theta$ such that $2sc-\sqrt 3c^2+\sqrt 3s^2=0$, so

\begin{align} \ & 2sc-\sqrt 3c^2+\sqrt 3s^2=0 \\ \ \implies & \sin (2\theta)- \sqrt 3 \cos (2\theta)=0 \\ \ \implies & 2\theta = \frac \pi 3\\ \ \implies & \theta = \frac \pi 6 \end{align}

If $\theta = \frac \pi 6$ then $\cos \theta = \frac{\sqrt 3}{2}$ and $\sin\theta = \frac 12$, so

$$\biggl(\frac 34+\frac 34 \biggr)X^2+\biggl(\frac 14 - \frac 34 \biggr)Y^2+\biggl(\frac 32 -\frac 12 \biggr)X+ \biggl(\frac{\sqrt 3}{2}+\frac{\sqrt 3}{2} \biggr)Y=\frac 73$$ $$\frac 32 X^2- \frac 12 Y^2+X+\sqrt 3 Y=\frac 73$$ $$\frac 32 \biggl(X+\frac 13 \biggr)^2- \frac 12 \bigl(Y-\sqrt 3 \bigr)^2=1$$

And hence $$\frac{\hat X ^2}{(\sqrt {2/3})^2}- \frac{\hat Y ^2}{(\sqrt 2)^2}=1$$

where $\hat X = X+\frac 13$ and $\hat Y = Y-\sqrt 3$