五、圆圆的面积( 1) ??????????????????????????????????????????????????2 C ????????= ×r 2?r2 S = ×r C2=?r×r 11=?r 2 ??????????????????????????????????????? ÷ ÷2 = 40 ÷2 = 20 (cm) (1)??????????2????????????????? 1256 ????? × 20 2 = × 400 = 1256 (cm 2) × 20 2 = × 400 = 1256 (cm 2) ?????????? × ( ÷ ÷ 2) 2 = × 20 2 = × 400 = 1256 (cm 2) × ( ÷ ÷ 2) 2 = × 20 2 = × 400 = 1256 (cm 2)???????????????????????????????????????? ×( ÷ ÷ 2) 2 = × 20 2 = × 400 = 1256 (cm 2) ×( ÷ ÷ 2) 2 = × 20 2 = × 400 = 1256 (cm 2) ×( 20 ÷ 2) 2= × 10 2= × 100 = 314 (m 2) ×( 20 ÷ 2) 2= × 10 2= × 100 = 314 (m 2)1 ????????????? S= ?(d÷2) 2 S= ?(C÷π÷2) 2 ???????????????? S= ?r 2 S= ?( ) 22 d S= ?(C÷?÷ 2) 2知道半径: 知道直径: 知道周长: S= ?(d÷2) 2 ?????????????????????????????????????×????× ??????????????????????× = ??× ??????*知识拓展*
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