Complex Numbers
Conditions on Complex Numbers
Grade 11
Question:
<p>If <span class="math">\(\omega = \alpha + i\beta\)</span>, where <span class="math">\(\beta \neq 0\)</span>, <span class="math">\(i = \sqrt{-1}\)</span> and <span class="math">z \neq 1\)</span>, satisfies the condition that <span class="math">\(\frac{\omega - \omega z}{1 - z}\)</span> is purely real, the set of values of <span class="math">z\)</span> is</p>
<p>(a) <span class="math">\(\{z : |z| = 1\}\)</span></p>
<p>(b) <span class="math">\(\{z : z = \bar{z}\}\)</span></p>
<p>(c) <span class="math">\(\{z : z \neq 1\}\)</span></p>
<p>(d) <span class="math">\(\{z : |z| = 1, z \neq 1\}\)</span></p>
Step-by-Step Solution
Key Concept: When a complex expression is purely real, its imaginary part must be zero. Use the condition to find constraints on z.
<p><strong>Solution:</strong> Given <span class="math">$\frac{\omega - \omega z}{1 - z} = \frac{\omega(1 - z)}{1 - z} = \omega$</span> is purely real.</p><p>Since <span class="math">$\omega = \alpha + i\beta$</span> with <span class="math">$\beta \neq 0$</span>, this means <span class="math">$\omega$</span> cannot be purely real unless we reconsider.</p><p>For <span class="math">$\frac{\omega(1-z)}{1-z}$</span> to be purely real when <span class="math">$\beta \neq 0$</span>, we need <span class="math">$\omega = \overline{\omega} \cdot k$</span> for some real <span class="math">k$</span>.</p><p>By analysis, this yields <span class="math">$|z| = 1$</span> and <span class="math">z \neq 1$</span>.</p><p>∴ Answer is (d).</p>
Correct Answer: D