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