A general development of solutions of Pell's equation x^2-ny^2 = 1 in terms of continued fractions for sqrt{n} can be presented. as the solutions x and y are approximates to the square root of n and thus are a special case of continued fraction approximations for quadratic irrationals.
The relationship to the continued fractions implies that the solutions to Pell's equation form a semigroup subset of the modular group. Thus, for example, if p and q satisfy Pell's equation, then