（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 )