0.1÷3.14≈0.0318半径: √0.0318≈0.178毫米直径: 0.178×2＝0.356毫米
S=兀r²，∴r=√（S/兀）=√（0.1/3.14）≈√0.0318=0.178
兀r^2＝0.1r^2=0.1/兀r＝√0.1/兀直径＝2r＝2×√0.1/兀
