\[\begin{array}{l} {\text{а) }}\sqrt {x + \frac{1}{2}\sqrt {x + \frac{1}{4}\sqrt {x + \frac{1}{8}\sqrt {x + \frac{1}{{16}}\sqrt {x + ...} } } } } = \frac{1}{4} + \sqrt x ; \hfill \\ {\text{б) }}\sqrt {x - \frac{1}{2}\sqrt {x - \frac{1}{4}\sqrt {x - \frac{1}{8}\sqrt {x - \frac{1}{{16}}\sqrt {x - ...} } } } } = - \frac{1}{4} + \sqrt x . \hfill \\ \end{array}\]