History of Hindu Mathematics: A Source Book v2.c17.s16.9

Also $k₁ = (n² - N)/k = (a₁² k - 2ba₁ + 1)/(a²)$ $= (a₁²(b² - Na²) - 2ba₁ + 1)/(a²)$ $= ((a₁ b - 1)/a)² - Na₁².$