Abstract : The relations between approximants of the regular continued fraction expansion for square root of C + L and the approximations given by an extension of Newton's formula are given. The general relationship is given between the approximants of a regular periodic continued fraction expansion for square root of C + L and the approximations given by the extended Newton's formula. These formulas are modified to the case of symmetric periodic continued fraction expansions. Rules are given that relate the approximants of mixed periodic continued fraction expansions for square root of C + L to approximations given by the extended Newton's formula. (Author)