System of polynomial equations.

Algebra Level 3

Given any real number c 0 c\neq 0 , the following system can be defined x 3 3 x y 2 + 3 c y 2 3 c x 2 + 4 c 2 x 2 c 3 = 0 y 3 + 3 x 2 y 6 c x y + 4 c 2 y = 0 \begin{aligned} x^3-3xy^2+ 3cy^2-3cx^2+4c^2x-2c^3 &=0\\ -y^3+3x^2y-6cxy+4c^2y &=0\end{aligned} Find the exact number of solutions ( x , y ) , (x, y), where x x and y y are real numbers and y 0. y\neq 0.


The answer is 2.

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2 solutions

Arturo Presa
Dec 14, 2018

The given system is equivalent to the polynomial equation z 3 3 c z 2 + 4 c 2 z 2 c 3 = 0 , z^3-3cz^2+4c^2z-2c^3=0, where z = x + i y . z=x+iy. Since the equation has the solutions c , c + c i c, c+c i and c c i , c-ci, then the corresponding solutions of the system are the pairs ( c , 0 ) , (c, 0), ( c , c ) (c, c) and ( c , c ) , (c, -c), and, therefore, the answer is 2.

The equations: c 0 y 0 2 c 3 + 4 x c 2 3 x 2 c + 3 y 2 c + x 3 3 x y 2 = 0 y 3 + 4 c 2 y + 3 x 2 y 6 c x y = 0 Over C [ x c y c x c y c x ( 1 i 2 ) c y c 2 x ( 1 i 2 ) c y c 2 x ( 1 + i 2 ) c y c 2 x ( 1 + i 2 ) c y c 2 ] Over R [ x c y c 2 x c y c 2 ] \begin{array}{l} c\neq 0 \\ y\neq 0 \\ -2 c^3+4 x c^2-3 x^2 c+3 y^2 c+x^3-3 x y^2=0 \\ -y^3+4 c^2 y+3 x^2 y-6 c x y=0 \\ \end{array} \Longrightarrow \begin{array}{ll} \text{Over}\ \mathbb{C} & \left[\begin{array}{ll} x\to c & y\to -c \\ x\to c &y\to c\ \\ x\to \left(1-\frac{i}{2}\right) c & y\to -\frac{c}{2} \\ x\to \left(1-\frac{i}{2}\right) c & y\to \frac{c}{2} \\ x\to \left(1+\frac{i}{2}\right) c & y\to -\frac{c}{2} \\ x\to \left(1+\frac{i}{2}\right) c &y\to \frac{c}{2} \\ \end{array}\right] \\ \text{Over}\ \mathbb{R} & \left[ \begin{array}{ll} x\to c & y\to -\sqrt{c^2} \\ x\to c & y\to \sqrt{c^2} \\ \end{array} \right] \\ \end{array}

In either case, there are 2 real solutions.

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