设圆心角为a,半径为r,高为h
cosa/2=h/r
a=2arccos(h/r)
把a转化成弧度：a=2arccos(h/r)*π/180°
则弧长L=2arccos(h/r)*π/180°
S=1/2Lr
=1/2*2arccos(h/r)*π/180°*r
=r*arccos(h/r)*π/180°