圆的面积课件.ppt

圆的面积,木棠中心校何贵山,学习目标1、理解圆面积的含义和圆面积计算公式的推导过程，掌握圆面积的计算公式。2、培养自己动手操作、抽象概括的能力，运用所学知识解决简单实际问题。3、学会渗透转化的数学思想。,一个圆的半径是r,它的周长的一半是（）,一个圆的半径是r,它的周长的一半是（）,r,学过的平面图形及它们的面积计算,S=ab,S=ah,学过的平面图形及它们的面积计算,学过的平面图形及它们的面积计算,S=ah2,学过的平面图形及它们的面积计算,S=(a+b)h2,圆所占平面的大小叫做圆的面积。,面积公式的推导,1,2,3,4,5,6,7,8,1,2,3,4,5,6,7,8,1,2,3,4,5,6,7,8,1,2,3,4,5,6,7,8,1,2,3,4,5,6,7,8,1,2,3,4,5,6,7,8,1,2,3,4,5,6,7,8,1,2,3,4,5,6,7,8,1,2,3,4,5,6,7,8,近似平行四边形,1,2,3,4,5,6,7,8,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,1,近似平行四边形,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,1,32等份,1,2,32,3,4,5,6,7,8,9,10,13,12,11,14,15,16,17,18,19,20,21,22,23,24,25,29,28,27,26,30,31,1,2,32,3,4,5,6,7,8,9,10,13,12,11,14,15,16,17,18,19,20,21,22,23,24,25,29,28,27,26,30,31,1,2,32,3,4,5,6,7,8,9,10,13,12,11,14,15,16,17,18,19,20,21,22,23,24,25,29,28,27,26,30,31,1,2,32,3,4,5,6,7,8,9,10,13,12,11,14,15,16,17,18,19,20,21,22,23,24,25,29,28,27,26,30,31,1,2,32,3,4,5,6,7,8,9,10,12,11,14,15,16,17,18,19,20,21,23,24,25,29,28,27,26,31,13,22,30,1,2,32,3,4,5,6,7,8,9,10,11,14,15,16,17,18,19,20,21,23,24,25,28,27,26,31,13,22,30,12,29,1,2,32,3,4,5,6,7,8,9,10,11,14,15,16,17,18,19,20,21,23,24,25,28,27,26,31,13,22,30,12,29,继续,1,2,32,3,4,5,6,7,8,9,10,11,14,15,16,17,18,19,20,21,23,24,25,28,27,26,31,13,22,30,12,29,8等份,16等份,32等份,分的份数越多，每一份越小，拼成的图形就越接近长方形,讨论：1、近似长方形的长与圆的周长有什么关系？2、近似长方形的宽与圆的半径有什么关系？,*,继续,继续,即：,结论：近似长方形的长=圆的周长一半,a=r,b=r,长方形面积=长宽,圆面积=,圆周长一半,半径,近似长方形的宽=圆的半径,s,=r,r,例题：,答：这个圆的面积是314平方米。,402=20（米）,答：这个圆的面积是1256平方米。,=3.14100,=314（平方米）,=3.14400,=1256（平方米）,列式计算：1、一个雷达圆形屏幕的半径是20厘米。它的面积是多少平方厘米？,答：它的面积是1256平方厘米。,2、一个圆形茶几桌面的直径是1米。它面积是多少？,12=0.5（米）,答：它的面积是50.24平方米,3、一种自动旋转喷灌装置的射程是10米，它能喷灌的面积是多少平方米？,=3.14,答：它的面积是314平米。,填空题：（1）如果一个圆的半径扩大3倍，它的直径扩大（）倍，面积