用余弦定理.C為45°或135°
a^4+b^4+c^4=2*c*c(a*a+b*b)
a^4+b^4+c^4-2a*a*c*c-2b*b*c*c=0
a^4+b^4+c^4-2a*a*c*c-2b*b*c*c+2a*a*b*b=2a*a*b*b
(a^2+b^2-c^2)^2=2a*a*b*b等式兩邊同時(shí)除以4a*a*b*b
cos^2C=1/2
∴cosC=√2/2 或-√2/2