a^2+b^2-c^2+2ab
--------------- =
a^2-b^2-c^2-2bc
=[(a+b)^2-c^2]/[a^2-(b+c)^2]
=(a+b+c)(a+b-c)/(a+b+c)(a-b-c)
=(a+b-c)/(a-b-c)