求值 a=r*arcsin(501/r),a为已知数,求r ,急用,

求值 a=r*arcsin(501/r),
a为已知数,求r ,急用,

a/r=arcsin(501/r)
sin(a/r)=sin(arcsin(501/r))
sin(a/r)=sin(501/r)
a/r=501/r+2pai
a=501+2pair
r=(a-501)/2pai