c^2=a^2+b^2-2abcosC,
cosC=(a^2+b^2-c^2)/2ab a^2+b^2=2c^2
=c^2/2ab 2ab≤a^2+b^2=2c^2
cosC≥c^2/2c^2=1/2 a=b时取最小值
cosC的最小值为1/2