A Pell's equation is given in the following way:
$ nx^2 + 1 = y^2 $
According to mathematical rules and the website http://www-groups.dcs.st-and.ac.uk/~history/HistTopics/Pell.html it can also be written like this:
$y^2 - nx^2 = 1$
The equation Bhaskara II uses as an example is:
$y^2 - 61x^2 = 1$
So you have to find x and y. A solution I found was $x = 226153980, y = 1766319049$. I tested the correctness of the result with the first version of the Pell's equation (see above):
$ 61x^2 + 1 = y^2 $
$ 61 * 226153980 + 1 = 1766319049^2 \Rightarrow y = 1766319049 $
So the result is correct. Now let's try it with the second way of writing it:
$ y^2 - 61x^2 = 1 $
$ 1766319049^2 - 61 * 226153980^2 = 0 \neq 1 $ (According to Google)
So the first equation proves the correctness and the second one? What's wrong with my logic or approach that the obviously correct solution equals 0?