(0,0)$? Give an equation a line other than $y=2x$ that
      passes through the origin. Then compute
      $$\ds\lim_{\substack{(x,y)\to(0,0)\\ \text{your line}
        }}\frac{x^2-y^2}{x^2+y^2} = \lim_{x\to 0}
        \frac{x^2-(?)^2}{x^2+(?)^2}=\ldots.$$
      </li>

      <li>Give another equation a line that passes through the
      origin. Then compute $$\ds\lim_{\substack{(x,y)\to(0,0)\\ \text{your line}}}\frac{x^2-y^2}{x^2+y^2}.$$
      </li>

      <li>Does this function have a limit at $(0,0)$? Exp