c^4-2(a^2+b^2)c^2+a^4+a^2b^2+b^4=0
c^4-2(a^2+b^2)c^2+a^4+2a^2b^2+b^4=a^2b^2
c^4-2(a^2+b^2)c^2+(a^2+b^2)^2=a^2b^2
(a^2+b^2-c^2)^2=a^2b^2
a^2+b^2-c^2=正负ab=2ab cosC
cosC=正负1/2
C=60°或120°
c^4-2(a^2+b^2)c^2+a^4+a^2b^2+b^4=0
c^4-2(a^2+b^2)c^2+a^4+2a^2b^2+b^4=a^2b^2
c^4-2(a^2+b^2)c^2+(a^2+b^2)^2=a^2b^2
(a^2+b^2-c^2)^2=a^2b^2
a^2+b^2-c^2=正负ab=2ab cosC
cosC=正负1/2
C=60°或120°