# [ASK] Make x and y the Subjects: x^3-3xy^2=a, 3x^2y-y^3=b

• MHB
• Monoxdifly
Monoxdifly
MHB
$$\displaystyle x^3-3xy^2=a$$
$$\displaystyle 3x^2y-y^3=b$$
make a formula using a and b for x and y.

What I've done:
$$\displaystyle x^3-3xy^2+3x^2y-y^3=a+b$$
$$\displaystyle x^3+3x^2y-3xy^2-y^3=a+b$$
$$\displaystyle (x-y)(x^2+4xy+y^2)=a+b$$
I don't know what to do from here. Any hints?

Gold Member
MHB
$$\displaystyle x^3-3xy^2=a$$
$$\displaystyle 3x^2y-y^3=b$$
make a formula using a and b for x and y.

What I've done:
$$\displaystyle x^3-3xy^2+3x^2y-y^3=a+b$$
$$\displaystyle x^3+3x^2y-3xy^2-y^3=a+b$$
$$\displaystyle (x-y)(x^2+4xy+y^2)=a+b$$
I don't know what to do from here. Any hints?
It looks to me as though this question is designed to be tackled using complex numbers. In fact, if you add $i$ times the second equation to the first equation, you get $x^3-3xy^2 + i(3x^2y-y^3) = a + ib$, which can be written $(x+iy)^3 = a + ib$. So $x+iy = (a+ib)^{1/3}$, and the solutions for $x$ and $y$ are then $$x = \operatorname{Re}(a+ib)^{1/3}, \qquad y = \operatorname{Im}(a+ib)^{1/3}.$$ Since a complex number has three cube roots, those formulas give three solutions for $x$ and $y$.

Monoxdifly
MHB
It looks to me as though this question is designed to be tackled using complex numbers. In fact, if you add $i$ times the second equation to the first equation, you get $x^3-3xy^2 + i(3x^2y-y^3) = a + ib$, which can be written $(x+iy)^3 = a + ib$. So $x+iy = (a+ib)^{1/3}$, and the solutions for $x$ and $y$ are then $$x = \operatorname{Re}(a+ib)^{1/3}, \qquad y = \operatorname{Im}(a+ib)^{1/3}.$$ Since a complex number has three cube roots, those formulas give three solutions for $x$ and $y$.

Complex number, eh? So, it's definitely not for high-schoolers, then.

skeeter
If the equations were ...

$$\displaystyle x^3+3xy^2=a$$
$$\displaystyle 3x^2y+y^3=b$$

... then the problem would fit the level of this forum.

Monoxdifly
MHB
If the equations were ...

$$\displaystyle x^3+3xy^2=a$$
$$\displaystyle 3x^2y+y^3=b$$

... then the problem would fit the level of this forum.

Guess I better change the question to that and make the answer $$\displaystyle x+y=\sqrt{a+b}$$.

skeeter
... could also be $x-y=\sqrt{a-b}$

Monoxdifly
MHB
Ugh... If only that$$\displaystyle 3xy^2$$ on the question was positive, this would be easy to answer.