Problem 147
Hard Conics and Transformations

By translating the coordinate axes, reduce to canonical form…

By translating the coordinate axes, reduce to canonical form and identify: $1^\circ$ $x^2+y^2-2x+4y+1=0$; $2^\circ$ $9x^2+4y^2+54x-8y+49=0$; $3^\circ$ $4x^2-16y^2+4x+16y-19=0$.

Explanation of Method

Moving the origin to the centre reduces a circle, an ellipse and a hyperbola to canonical form.

Step 1: Translation of the axes

A translation of the axes to the new origin $O_1(x_0,y_0)$ is

\[ x=x'+x_0,\qquad y=y'+y_0 . \]

We choose $O_1$ by completing the squares; then the linear terms vanish.

Step 2: $1^\circ$ Circle

\[ (x^2-2x+1)+(y^2+4y+4)=-1+1+4\ \Longrightarrow\ (x-1)^2+(y+2)^2=4. \]

With $O_1(1,-2)$, i.e. $x=x'+1,\ y=y'-2$: $x'^2+y'^2=4:\quad\text{circle, centre }(1,-2),\ r=2$.

Step 3: $2^\circ$ Ellipse

\[ 9(x^2+6x+9)+4(y^2-2y+1)=-49+81+4=36 \]

\[ 9(x+3)^2+4(y-1)^2=36\ \Longrightarrow\ \frac{(x+3)^2}{4}+\frac{(y-1)^2}{9}=1. \]

With $O_1(-3,1)$: $\dfrac{x'^2}{4}+\dfrac{y'^2}{9}=1$. Here $b=3>a=2$, so the major axis is parallel to the $y$-axis; $c=\sqrt{9-4}=\sqrt5$, $e=\sqrt5/3$.

\[\frac{x'^2}{4}+\frac{y'^2}{9}=1:\quad\text{ellipse, centre }(-3,1),\ F_{1,2}(-3,\,1\pm\sqrt5)\]

Step 4: $3^\circ$ Hyperbola

\[ 4\left(x^2+x+\tfrac14\right)-16\left(y^2-y+\tfrac14\right)=19+1-4=16 \]

\[ 4\left(x+\tfrac12\right)^2-16\left(y-\tfrac12\right)^2=16\ \Longrightarrow\ \frac{\left(x+\frac12\right)^2}{4}-\left(y-\tfrac12\right)^2=1. \]

With $O_1\left(-\tfrac12,\tfrac12\right)$: $a=2$, $b=1$, $c=\sqrt5$, asymptotes $y'=\pm\tfrac12x'$.

\[\frac{x'^2}{4}-\frac{y'^2}{1}=1:\quad\text{hyperbola, centre }\left(-\tfrac12,\tfrac12\right)\]

x y −1 1 2 3 −4 −3 −2 −1 1 $x'$ $y'$ $r=2$ $O_1$
$1^\circ$ Circle: centre $O_1(1,-2)$, $r=2$; $x'^2+y'^2=4$.
x y −6 −5 −4 −2 −1 −2 −1 1 2 3 4 $x'$ $y'$ $F_1$ $F_2$ $O_1$
$2^\circ$ Ellipse: centre $(-3,1)$, $a=2$, $b=3$; the major axis is vertical, foci $(-3,1\pm\sqrt5)$.
x y −4 −2 2 4 2 $x'$ $y'$ $O_1$
$3^\circ$ Hyperbola: centre $\left(-\tfrac12,\tfrac12\right)$, $a=2$, $b=1$; asymptotes (orange) have slopes $\pm\tfrac12$.

Result

\[\boxed{\,x'^2+y'^2=4:\quad\text{circle, centre }(1,-2),\ r=2\,}\]

\[\boxed{\,\frac{x'^2}{4}+\frac{y'^2}{9}=1:\quad\text{ellipse, centre }(-3,1),\ F_{1,2}(-3,\,1\pm\sqrt5)\,}\]

\[\boxed{\,\frac{x'^2}{4}-\frac{y'^2}{1}=1:\quad\text{hyperbola, centre }\left(-\tfrac12,\tfrac12\right)\,}\]