title: Complex numbers product using only three multiplications
body: <p>We do complex number multiplication as follows:</p>
  
  <pre><code>(a + i * b) * (c + i * d) = (a * c - b * d) + i * (a * d + b * c)
  </code></pre>
  
  <p>The real and imaginary parts of the result are</p>
  
  <pre><code>real part = (a * c - b * d)
  imag part = (a * d + b * c)
  </code></pre>
  
  <p>This involves four real multiplications. How can we do it with only three real multiplications?</p>