(a^2-c^2)^2=b^2(2c^2-b^2)
a^4+c^4+b^4-2a^2c^2-2b^2c^2=0
a^4+c^4+b^4-2a^2c^2-2b^2c^2+2a^2b^2=2a^2b^2
(a^2+b^2-c^2)^2=2a^2b^2
a^2+b^2-c^2=（根号2）ab
cosC=a^2+b^2-c^2/2ab=根号2/2
所以C＝45度