4.5cm50.24c 8cm63.585c 3cm28.26c 40cm1256c 大圆周长:3.1410=31.4(cm)大圆面积:3.14 (102 )2=78.5(c )小圆周长:23.143=18.84(cm)小圆面积:3.1432=28.26(c )3.14102=314( )树干的半径:125.63.142= 20(cm)树干的横截面面积:3.14202=1256(c )圆环面积公式:S= (R2-r2)大圆半径:182=9(cm ) 综合算式:3.14 (182 )2 3.14 (72 )2=215.875(c )小圆半径:72=3.5(cm ) 圆环面积:3.14(92-3.52)=215.875(c )阴影部分面积:S= R2- r2小圆半径:62=3(cm ) 3.1462 -3.1432=84.78(c )周长:83.142 123.142 (12-8)=35.4(cm) 面积:3.14(122)2-3.14(82)2=62.8 (c ) 62.82=31.4 ( c )周长:23.148 23.1412=125.6(cm) 面积:3.14(122-82) =2