If the line joining origin to the points of intersection of the line fx - gy = λ and the curve x2+hxy−y2+gx+fy=0 be mutually perpendicular, then
Making the equation of curve homogeneous with the help of equation of line fx−gyλ=1 and to be perpendicular to both the lines represented by this homogeneous equation.
a + b = 0 ⇒λ+gf−λ−gf=0⇒0=0
Hence, λ may have any value.