x^2+y^2=9
x+y=2
2x+y=13
x^2+xy-30=0
\[ a_k = \left( \frac{k \cdot 3^k}{2} \right) \left( \frac{-1k}{3k} \right) = \left( \frac{k \cdot (-1)^k}{2} \right) \]
ax+a^2x+aby+by-(ax+by)^2
x^2+xy-30=0.2x+y=13,
(3x+6)(x-1)
2x+y=13,
8x+7y=4
3y-8x=36
\begin{cases} x+3y= 2 \\ x^2+y^2 = 2 \end{cases}
(x-1)(2x+9)
