)=(1,2)$, and you move
            to $(1.2, 1.7)$, then what is $dx$ and $dy$?
            Approximate how much $z=f(x,y)$ changes.</li>

            <li>(Challenge) In the last part, why was our
            calculation an approximation, rather than an exact
            calculation of how much $f(x,y)$ changed?</li>
    </ol>
  </div>

            
                    
                    
                    <p>We need to add some vocabulary to make it easier to talk
  about what we just did. Let's introd