General simplification
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$$5 x^{2} + x y - y^{2}$$
The perfect square
Let's highlight the perfect square of the square three-member
$$5 x^{2} + \left(x y - y^{2}\right)$$
Let us write down the identical expression
$$5 x^{2} + \left(x y - y^{2}\right) = - \frac{21 y^{2}}{20} + \left(5 x^{2} + x y + \frac{y^{2}}{20}\right)$$
or
$$5 x^{2} + \left(x y - y^{2}\right) = - \frac{21 y^{2}}{20} + \left(\sqrt{5} x + \frac{\sqrt{5} y}{10}\right)^{2}$$
in the view of the product
$$\left(- \sqrt{\frac{21}{20}} y + \left(\sqrt{5} x + \frac{\sqrt{5}}{10} y\right)\right) \left(\sqrt{\frac{21}{20}} y + \left(\sqrt{5} x + \frac{\sqrt{5}}{10} y\right)\right)$$
$$\left(- \frac{\sqrt{105}}{10} y + \left(\sqrt{5} x + \frac{\sqrt{5}}{10} y\right)\right) \left(\frac{\sqrt{105}}{10} y + \left(\sqrt{5} x + \frac{\sqrt{5}}{10} y\right)\right)$$
$$\left(\sqrt{5} x + y \left(- \frac{\sqrt{105}}{10} + \frac{\sqrt{5}}{10}\right)\right) \left(\sqrt{5} x + y \left(\frac{\sqrt{5}}{10} + \frac{\sqrt{105}}{10}\right)\right)$$
$$\left(\sqrt{5} x + y \left(- \frac{\sqrt{105}}{10} + \frac{\sqrt{5}}{10}\right)\right) \left(\sqrt{5} x + y \left(\frac{\sqrt{5}}{10} + \frac{\sqrt{105}}{10}\right)\right)$$
/ / ____\\ / / ____\\
| y*\-1 + \/ 21 /| | y*\1 + \/ 21 /|
|x - ---------------|*|x + --------------|
\ 10 / \ 10 /
$$\left(x - \frac{y \left(-1 + \sqrt{21}\right)}{10}\right) \left(x + \frac{y \left(1 + \sqrt{21}\right)}{10}\right)$$
(x - y*(-1 + sqrt(21))/10)*(x + y*(1 + sqrt(21))/10)
Assemble expression
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$$5 x^{2} + x y - y^{2}$$
$$5 x^{2} + x y - y^{2}$$
Combining rational expressions
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$$5 x^{2} + y \left(x - y\right)$$
$$5 x^{2} + x y - y^{2}$$
$$5 x^{2} + x y - y^{2}$$
Rational denominator
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$$5 x^{2} + x y - y^{2}$$
$$5 x^{2} + x y - y^{2}$$