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4.3 圆面积 PowerPoint PPT Presentation


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4.3 圆面积. 期末倒计时. 把一个圆 12 等分. 对半分. 半个圆先拼接. 另外半个继续拼接. 最后一片处理一下就成. 对半开. (注: C=2πr )半个圆周长 πr. 6. 1. 2. 3. 4. 5. r. 圆的面积近似可以看成长方形面积. S=πrxr=πr 2. 圆的半径扩大 3 倍,面积扩大几倍? 假设原来的半径为 r , 那么 S=π r 2 扩大三倍后半径为 3r ,那么 S=πx3rx3r=9πr 2 ∴ 是原来的 9 倍。 那么直径扩大 3 倍呢?

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4.3 圆面积

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4.3圆面积


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期末倒计时

把一个圆12等分


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对半分


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半个圆先拼接


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另外半个继续拼接

最后一片处理一下就成


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对半开


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(注:C=2πr)半个圆周长πr

6

1

2

3

4

5

r


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圆的面积近似可以看成长方形面积

S=πrxr=πr2


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圆的半径扩大3倍,面积扩大几倍?

假设原来的半径为r ,那么S=π r 2

扩大三倍后半径为3r,那么S=πx3rx3r=9πr 2

∴是原来的9倍。

那么直径扩大3倍呢?

【结论】圆面积S与圆的半径r或者直径d扩大、缩小的倍数成平方关系。


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环形面积=大圆面积-小圆面积

设大圆半径R、小圆半径r

S圆环=πR2– π r2 =πx(R2- r2 )


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已知甲圆的半径长是乙圆的直径长,且他们的面积之和是100cm2

那么甲圆的面积是多少?


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RESUME


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