1985 AHSME Problems/Problem 23

Problem

If $x=\frac{-1+i\sqrt{3}}{2}$ and $y=\frac{-1-i\sqrt{3}}{2}$ , where $i^2=-1$ , then which of the following is not correct?

$\mathrm{(A)\ } x^5+y^5=-1 \qquad \mathrm{(B) \ }x^7+y^7=-1 \qquad \mathrm{(C) \ } x^9+y^9=-1 \qquad$

$\mathrm{(D) \ } x^{11}+y^{11}=-1 \qquad \mathrm{(E) \ }x^{13}+y^{13}=-1$

Notice that $x+y=-1$ and $xy=1$ . We have $1=(x+y)^2=x^2+2xy+y^2=x^2+y^2+2\implies x^2+y^2=-1$ .

We also have $x^3+y^3=(x+y)^3-3x^2y-3xy^2=(-1)^3-3xy(x+y)=-1-3(1)(-1)=2$ .

Finally, $x^5+y^5=(x+y)^5-5x^4y-5xy^4-10x^3y^2-10x^2y^3$

$=(-1)^5-5xy(x^3+y^3+2xy(x+y))=-1-5(1)(2+2(1)(-1))=-1$ .

Let $x^{2n+1}+y^{2n+1}=z$ . We have

$-z=(x^{2n+1}+y^{2n+1})(x^2+y^2)$

$-z=x^{2n+3}+y^{2n+3}+x^2y^{2n+1}+x^{2n+1}y^2$

$-z=x^{2n+3}+y^{2n+3}+(xy)^2(x^{2n-1}+y^{2n-1})$

$-z=x^{2n+3}+y^{2n+3}+x^{2n-1}+y^{2n-1}$ .

For $n=2, z=-1$ , so

$1=x^7+y^7+x^3+y^3=x^7+y^7+2\implies x^7+y^7=-1$ .

Therefore, for $n=3, z=-1$ , and

$1=x^9+y^9+x^5+y^5=x^9+y^9-1\implies x^9+y^9=2$ , so $x^9+y^9\not=-1, \boxed{\text{C}}$ .

As a check, we have $z=2$ for $n=4$ , and

$-2=x^{11}+y^{11}+x^7+y^7=x^{11}+y^{11}-1\implies x^{11}+y^{11}=-1$ .

Finally, for $n=5$ we have $z=-1$ , and

$1=x^{13}+y^{13}+x^9+y^9=x^{13}+y^{13}+2\implies x^{13}+y^{13}=-1$ , and this is true for all other answer choices.

1985 AHSME (Problems • Answer Key • Resources)
Preceded by Problem 22	Followed by Problem 24
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