等腰三角形腰长500,底边715,求顶角?底边是720时求顶角?

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……

设顶角为2a,则
sina=715/(2*500)=0.715,
∴a=arcsin0.715,
顶角为2arcsin0.715.

从顶点往底边做垂直线，底边的一半=715/2=357.5
设角度为2x
则sin x=357.5/500=0.715
x=arcsin 0.715
2x=2arcsin 0.715
底边是720时同理

arcsin(715/1000) = 45.60
arcsin(720/1000) = 46.05