  In this paper the properties of R\'edei rational functions are used to derive
rational approximations for square roots and both Newton and Pad\'e
approximations are given as particular cases. As a consequence, such
approximations can be derived directly by power matrices. Moreover, R\'edei
rational functions are introduced as convergents of particular periodic
continued fractions and are applied for approximating square roots in the field
of p-adic numbers and to study periodic representations. Using the results over
the real numbers, we show how to construct periodic continued fractions and
approximations of square roots which are simultaneously valid in the real and
in the p-adic field.
