Skip to main content

[Community Question] Algebra-Precalculus: Find all integer solutions to $ x^3 - y^3 = 3(x^2 - y^2) $

One of our user asked:

The objective is to find all solutions to $$ x^3 - y^3 = 3(x^2 - y^2) $$ where $x,y \in \mathbb{Z}$.


So far I've got one pair of solution. Try $(x, y)=(0,0)$: $$ 0^3-0^3=3(0^2-0^2) \\ 0=0 \qquad \text{equation satisfied}$$

Another try $ (x, y) = (x, x)$ then $$ x^3 - x^3 = 3(x^2 -x^2) \\ 0 = 0 \qquad \text{equation satisfied}$$

But the above are just particular solutions. To find all $(x, y)$ pairs, I tried:

$$\begin{aligned} (x-y)(x^2+xy+y^2) &= 3(x+y)(x-y) \\ x^2+xy+y^2 &= 3(x+y) \end{aligned}\\ \begin{aligned} x^2+xy+y^2-3x-3y &= 0 \\ x^2+x(y-3)+y(y-3) &=0 \\ x^2 + (y-3)(x+y) &= 0 \end{aligned}$$

What now? Am I on the right track?


Comments

Popular posts from this blog

[Community Question] Calculus: calculating 2 constants in a function

One of our user asked: $$M=\{f\in C[0,2\pi],\int_{0}^{2\pi}f(x)sinxdx=\pi,\int_{0}^{2\pi}f(x)sin2xdx=2\pi\} $$ $a,b\in \mathbb R, g\in M, g(x)=asinx+bsin2x,x\in [0,2\pi]$ I've read on the answers that $a=1,b=2$ and I don't know how to calculate them. Can somebody explain me,please? By the way, the problem is to determine $ \int_{0}^{2\pi}(g(x))^2 dx$ so you have to first get the constants $a$ and $b$ .

Calculus: A difficulty in understanding a step in a solution.

Here is the solution: But I could not understand how the last term in the fourth line came from the line before it, could anyone explain this for me please?