Maths Olympiad Prep

Library / /44 of 82

Algebra Difficulty 5.6 AIME, harder Prove it Croatia

Determine all complex numbers zz such that
z2i=1andz=2. |z^2 - i| = 1 \quad \text{and} \quad |z| = \sqrt{2}.

Solution

Let z=x+iyz = x + iy, where x,yRx, y \in \mathbb{R}.

Given z=2|z| = \sqrt{2}, so
z2=x2+y2=2. |z|^2 = x^2 + y^2 = 2.

Also, z2i=1|z^2 - i| = 1.
Compute z2z^2:
z2=(x+iy)2=x2y2+2ixy. z^2 = (x + iy)^2 = x^2 - y^2 + 2ixy.
So,
z2i=(x2y2)+2ixyi=(x2y2)+i(2xy1). z^2 - i = (x^2 - y^2) + 2ixy - i = (x^2 - y^2) + i(2xy - 1).
Therefore,
z2i2=(x2y2)2+(2xy1)2=1. |z^2 - i|^2 = (x^2 - y^2)^2 + (2xy - 1)^2 = 1.

Now, we have the system:
\begin{align*}
x^2 + y^2 &= 2 \\
(x^2 - y^2)^2 + (2xy - 1)^2 &= 1
\end{align*}

Let us use polar form: z=reiθz = r e^{i\theta}, r=2r = \sqrt{2}, so z=2eiθz = \sqrt{2} e^{i\theta}.
Then z2=2ei2θz^2 = 2 e^{i2\theta}, so
z2i=2ei2θi. z^2 - i = 2 e^{i2\theta} - i.

Write 2ei2θ=2(cos2θ+isin2θ)2 e^{i2\theta} = 2(\cos 2\theta + i \sin 2\theta), so
z2i=2cos2θ+i(2sin2θ1). z^2 - i = 2\cos 2\theta + i(2\sin 2\theta - 1).

Therefore,
z2i2=(2cos2θ)2+(2sin2θ1)2=1. |z^2 - i|^2 = (2\cos 2\theta)^2 + (2\sin 2\theta - 1)^2 = 1.
Expand:
\begin{align*}
4\cos^2 2\theta + (4\sin^2 2\theta - 4\sin 2\theta + 1) &= 1 \\
4\cos^2 2\theta + 4\sin^2 2\theta - 4\sin 2\theta + 1 &= 1 \\
4(\cos^2 2\theta + \sin^2 2\theta) - 4\sin 2\theta + 1 &= 1 \\
4(1) - 4\sin 2\theta + 1 &= 1 \\
4 - 4\sin 2\theta + 1 = 1 \\
4 - 4\sin 2\theta = 0 \\
1 - \sin 2\theta = 0 \\
\sin 2\theta = 1
\end{align*}

So 2θ=π2+2πk2\theta = \frac{\pi}{2} + 2\pi k, kZk \in \mathbb{Z}.
Thus θ=π4+πk\theta = \frac{\pi}{4} + \pi k.

Therefore, the solutions are
z=2ei(π4+πk) z = \sqrt{2} e^{i\left(\frac{\pi}{4} + \pi k\right)}
for kZk \in \mathbb{Z}.

Explicitly, for k=0k = 0:
z1=2eiπ4=2(cosπ4+isinπ4)=2(22+i22)=1+i. z_1 = \sqrt{2} e^{i\frac{\pi}{4}} = \sqrt{2}\left(\cos \frac{\pi}{4} + i \sin \frac{\pi}{4}\right) = \sqrt{2}\left(\frac{\sqrt{2}}{2} + i \frac{\sqrt{2}}{2}\right) = 1 + i.
For k=1k = 1:
z2=2ei(5π4)=2(cos5π4+isin5π4)=2(22i22)=1i. z_2 = \sqrt{2} e^{i\left(\frac{5\pi}{4}\right)} = \sqrt{2}\left(\cos \frac{5\pi}{4} + i \sin \frac{5\pi}{4}\right) = \sqrt{2}\left(-\frac{\sqrt{2}}{2} - i \frac{\sqrt{2}}{2}\right) = -1 - i.

Thus, the solutions are z=1+iz = 1 + i and z=1iz = -1 - i.

Want a route through all this instead of an archive? The track puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.

Source: MathNet, licensed CC-BY-4.0. Statement and solution reproduced as published; topic and difficulty added by this site.