逗號:S=
1
2
×3.14×(2r)2=6.28r2;
問號:S=
3
4
×[3.14×(2r)2?3.14r2]+3.14r2=10.205r2;
因為10.205r2>9.42r2>6.28r2,
所以問號的油漆用得多.