可以用余旋定理.
定理如下:a^2+b^2-2abcosC=c^2
所以2abcosC(顶角)=a^2+b^2-c^2
500000cosC=250000+250000-511225
500000cosC=-11225
cosC=-0.02245
∠C=91.286398323498628310588058837439……
当底边是720时,
500000cosC=250000+250000-518400
500000cosC=-18400
cosC=-0.0368
∠C=92.108960875382327700571936783872……