圆锥表面积=∏r√（r^2+4R^2)+∏r^2
球面积=4∏r^2
则∏r√（r^2+4R^2)+∏r^2=4∏r^2
r^2=R^2/2
圆锥的体积
=∏r^2*2R/3=∏R^2/2 *2R/3=∏R^3/3