(^{page no 213 note *}) This result is often used. (Of course, it is simple to prove algebraically, but the geometry makes it obvious.) E.g. a standard method of transforming the integral ∫(ax ² + bx + c)^–1(a'x ² + b'x + c')^–½ dx, where b ² < 4ac, is to put x = (λ +μt)/(1 + t), where λ, μ are the roots of the jacobian of ax ² + bx + c and a'x ² + b'x + c'