阴S=(4²π/4-4²/2)/2 =(4π-8)/2 =4,56/2 =2,28
3.14×4×4×1/4-4×4/2 =3.14×4-8 =12.56-8 =4.56
1/4*π*4*4-1/2*4*4=4π-8
