(a^2+b^2-c^2)^2-4a^2b^2
=[(a^2+b^2-c^2)+2ab][(a^2+b^2-c^2)-2ab]
=(a²+2ab+b²-c²)(a²-2ab+b²-c²)
=(a+b-c)(a+b+c)(a-b+c)(a-b-c)
∵abc是三角形ABC的三边,
∴a+b-c>0
a+b+c>0
a-b+c>0,
a-b-c