<s>[INST] Transforming coordinates system to obtain an ellipse in the standard form Let $E$ be the ellipse $x^2+xy +y^2 = 1$. I would like to obtain the area of $E$ with the formula $ab\pi$. I am not able to transform the equation in the form $\dfrac{x^2}{a^2}+\dfrac{y^2}{b^2} = 1$. The textbook I am following uses a transformation to describe $E$ as $$\dfrac{X^2}{\left(\dfrac{1}{\sqrt{3}}\right)^2}+\dfrac{Y^2}{1^2}=1$$
I do not understand what intermediate steps lead to this transformation. 
EDIT : As some were curious, this is the original question: 
Use Stoke's Theorem to to evaluate $\oint_C\vec{F}\cdot\vec{dr}$. Assume C is oriented counterclockwise as viewed from above.
$$\vec{F} = z\vec{i}+x\vec{j} + y\vec{k}$$
$C$ is the curve of intersection of the plane $x + y + z = 0$ and the sphere $x^2 + y^2 + z^2 = 1$. 
Hint : recall that the area of an ellipse $\dfrac{x^2}{a^2}+\dfrac{y^2}{b^2} = 1$ is $ab\pi$
The textbook solution : 3 * Area of R = $3 \cdot \dfrac{\pi}{\sqrt(3)}$=$\sqrt3\pi$
 [/INST] Let $X=\dfrac x2$ and $Y=\dfrac x2  + y$.
Then $\dfrac{X^2}{\left(\dfrac{1}{\sqrt{3}}\right)^2}+\dfrac{Y^2}{1^2}=3X^2+Y^2=\dfrac34x^2+\dfrac14x^2+xy+y^2=1$.
Addendum:
The above explains the intermediate steps that led to the transformation, as the original post requested, but it should be noted, as alluded to in the comments, that the transformation 
$\begin{pmatrix}X\\Y\end{pmatrix}=\begin{pmatrix}\frac12 & 0 \\ \frac12 &1\end{pmatrix}\begin{pmatrix}x\\y\end{pmatrix}$ has determinant $\frac12$, so this transformation halves area, and that must be taken into account when addressing the underlying question about the area of the ellipse.
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