title: using lambda instead of let in scheme
body: <p>In SICP 1.2.1 there is a function that makes a rational number, as follow:</p>
  
  <pre><code>(define (make-rat n d)
    (let ((g (gcd n d)))
      (cons (/ n g) (/ d g))))
  </code></pre>
  
  <p>I'm just curious how you can implement the same thing using lambda instead of let, without calling GCD twice. I couldn't figure it out myself.</p>