History of Hindu Mathematics: A Source Book v2.c17.s14.39

v2.c17.s14.39

That is to say, if x = α, y = β be a solution of the equation Nx² + k = y², and x = α', y = β' be a solution of Nx² + k' = y², then, according to the above, x=αβ'±α'β, y=ββ'± Nαα' is a solution of the equation Nx²+kk'=y². In other words, if N\alpha²+k &= \beta², / N\alpha'²+k' &= \beta'², then N(αβ'±α'β)²+kk'=(ββ'± N.α')². I