A box open from top is made from a rectangular sheet of dimension \( a \times b \) by cutting squares each of side \( x \) from each of the four corners and folding up the flaps. If the volume of the box is maximum, then \( x \) is equal to :
\( \frac{a + b + \sqrt{a^{2} + b^{2} - ab}}{6} \)
\( \frac{a + b - \sqrt{a^{2} + b^{2} - ab}}{12} \)
\( \frac{a + b - \sqrt{a^{2} + b^{2} - ab}}{6} \)
\( \frac{a + b - \sqrt{a^{2} + b^{2} + ab}}{6} \)
