余弦定理推的.
c^2=b^2+a^2+2abcosC (1)等式两边同时平方
c^4=b^4+a^4+4a^b^2(cosC)^2+4(b^2+a^2)abcosC+2a^2b^2 (2)
在将(1)转变为2bccosC=c^2-(b^2+a^2) (3)
将(3)代入(2),得
c^4=b^4+a^4+4a^b^2(cosC)^2+2(b^2+a^2){c^2-(b^2+a^2)}+2a^2b^2
c^4=b^4+a^4+4a^b^2(cosC)^2+2(b^2+a^2)c^2-2b^4-2a^4-4a^2b^2+2a^2b^2
c^4+b^4+a^4-2(b^2+a^2)c^2+2a^2b^2-4a^b^2(cosC)^2=0 (4)
因此由标题等式和(4)得,4a^b^2(cosC)^2=3a^2b^2
(cosC)^2=3/4 =>cosC=±二分之根3,得角C=30或者150度.