General simplification
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$$- 9 a^{2} + 13 a y + y^{2}$$
The perfect square
Let's highlight the perfect square of the square three-member
$$- 9 a^{2} + \left(a 13 y + y^{2}\right)$$
Let us write down the identical expression
$$- 9 a^{2} + \left(a 13 y + y^{2}\right) = \frac{205 y^{2}}{36} + \left(- 9 a^{2} + 13 a y - \frac{169 y^{2}}{36}\right)$$
or
$$- 9 a^{2} + \left(a 13 y + y^{2}\right) = \frac{205 y^{2}}{36} - \left(3 a - \frac{13 y}{6}\right)^{2}$$
/ / _____\\ / / _____\\
| y*\13 - \/ 205 /| | y*\13 + \/ 205 /|
|a - ----------------|*|a - ----------------|
\ 18 / \ 18 /
$$\left(a - \frac{y \left(13 - \sqrt{205}\right)}{18}\right) \left(a - \frac{y \left(13 + \sqrt{205}\right)}{18}\right)$$
(a - y*(13 - sqrt(205))/18)*(a - y*(13 + sqrt(205))/18)
Assemble expression
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$$- 9 a^{2} + 13 a y + y^{2}$$
$$- 9 a^{2} + 13 a y + y^{2}$$
Rational denominator
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$$- 9 a^{2} + 13 a y + y^{2}$$
Combining rational expressions
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$$- 9 a^{2} + y \left(13 a + y\right)$$
$$- 9 a^{2} + 13 a y + y^{2}$$
$$- 9 a^{2} + 13 a y + y^{2}$$
$$- 9 a^{2} + 13 a y + y^{2}$$