<p><strong> Câu 3:</strong></p> <p>Tìm a để hàm số <span class="math-tex">$f(x) = \left\{ \begin{array}{l}\frac{{\sin (x - 1)}}{{\mathop x\nolimits^2 - 1}}\,\,\,\,\,\,\,\,\,\,\,\,\,\,(x \ne 1)\\a - \frac{1}{2}\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,(x = 1)\end{array} \right.$</span> liên tục tại x=1</p>