Triangles

Triangle is formed by the lines $x+y=0$ , $x-y=0$ and $lx+my=1$ . If $m$ and $l$ vary subject to the condition $l^2+m^2=1$ and the circumcentre of the triangle is $(h,k)$ , then find the value of $\dfrac{(h^2-k^2)^2}{h^2 + k^2}$ .