History of Hindu Mathematics: A Source Book v2.c17.s16.6

Now $n² - N = ((a₁ k - b)² - Na²)/(a²)$ $= (a₁² k² - 2 b k a₁ + k)/(a²)$ $= (k(a₁² k - 2 b a₁ + 1))/(a²).$